Use Windows Crypto API to calculate a MD5 string.

// cMD5.h: interface for the cMD5 class.

#if !defined(AFX_CMD5_H__6C2F7C61_93D8_11D8_A8BA_000AE637F271__INCLUDED_)
#define AFX_CMD5_H__6C2F7C61_93D8_11D8_A8BA_000AE637F271__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000

#define _ReadBufSize 1000000

class cMD5  
{
public:
    char* CalcMD5FromString(const char *s8_Input);
    char* CalcMD5FromFile  (const char *s8_Path);

    void FreeBuffer();
    cMD5();
    virtual ~cMD5();

private:
    struct MD5Context 
    {
        unsigned long buf[4];
        unsigned long bits[2];
        unsigned char in[64];
    };

    void MD5Init();
    void MD5Update(unsigned char *buf, unsigned len);
    void MD5Final (unsigned char digest[16]);
    
    void MD5Transform(unsigned long buf[4], unsigned long in[16]);
    char* MD5FinalToString();
    
    void byteReverse (unsigned char *buf, unsigned longs);

    char *mp_s8ReadBuffer;
    MD5Context ctx;
    char   ms8_MD5[40]; // Output buffer
};

#endif // !defined(AFX_CMD5_H__6C2F7C61_93D8_11D8_A8BA_000AE637F271__INCLUDED_)

#include "cMD5.h"

#ifdef _DEBUG
#undef THIS_FILE
static char THIS_FILE[]=__FILE__;
#define new DEBUG_NEW
#endif


cMD5::cMD5()
{
    // don't alloc buffer here, only on request
    mp_s8ReadBuffer = 0;
}

cMD5::~cMD5()
{
    FreeBuffer();
}

void cMD5::FreeBuffer()
{
    if (mp_s8ReadBuffer) delete mp_s8ReadBuffer;
}

/*********************************************************************
//
//         calculate MD5 from a file of any size (also size = 0)
//         returns "" on file error
//
/********************************************************************/

char* cMD5::CalcMD5FromFile(const char *s8_Path)
{
    if (!mp_s8ReadBuffer) mp_s8ReadBuffer = new char[_ReadBufSize];

    MD5Init();

    // ++++++++++++ Read file block by block +++++++++++

    HANDLE h_File = CreateFile(s8_Path, GENERIC_READ, FILE_SHARE_READ, 0, 
                               OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
        
    if (h_File == INVALID_HANDLE_VALUE)
        return "";

    int s32_Size = GetFileSize(h_File, 0);

    while (s32_Size > 0)
    {
        unsigned long u32_Read = 0;
        if (!ReadFile(h_File, mp_s8ReadBuffer, _ReadBufSize, &u32_Read, 0))
        {
            CloseHandle(h_File);
            return "";
        }

        MD5Update((unsigned char*)mp_s8ReadBuffer, u32_Read);

        s32_Size -= u32_Read;
    };

    CloseHandle(h_File);

    // ++++++++++++ Signature --> String +++++++++++

    return MD5FinalToString();
}


/*********************************************************************
//
//                calculate MD5 from a string
//
/********************************************************************/


char* cMD5::CalcMD5FromString(const char *s8_Input)
{
    MD5Init();
    MD5Update((unsigned char*)s8_Input, strlen(s8_Input));

    return MD5FinalToString();
}


/*********************************************************************
//
//                           Calculation functions
//
/*********************************************************************
/*
 * Start MD5 accumulation.  Set bit count to 0 and buffer to mysterious
 * initialization constants.
 */
void cMD5::MD5Init()
{
    ctx.buf[0] = 0x67452301;
    ctx.buf[1] = 0xefcdab89;
    ctx.buf[2] = 0x98badcfe;
    ctx.buf[3] = 0x10325476;

    ctx.bits[0] = 0;
    ctx.bits[1] = 0;
}

/*
 * Update context to reflect the concatenation of another buffer full
 * of bytes.
 */
void cMD5::MD5Update(unsigned char *buf, unsigned len)
{
    unsigned long t;

    /* Update bitcount */

    t = ctx.bits[0];
    if ((ctx.bits[0] = t + ((unsigned long) len << 3)) < t)
    ctx.bits[1]++;  /* Carry from low to high */
    ctx.bits[1] += len >> 29;

    t = (t >> 3) & 0x3f;    /* Bytes already in shsInfo->data */

    /* Handle any leading odd-sized chunks */

    if (t) {
    unsigned char *p = (unsigned char *) ctx.in + t;

    t = 64 - t;
    if (len < t) {
        memcpy(p, buf, len);
        return;
    }
    memcpy(p, buf, t);
    cMD5::byteReverse(ctx.in, 16);
    MD5Transform(ctx.buf, (unsigned long *) ctx.in);
    buf += t;
    len -= t;
    }
    /* Process data in 64-byte chunks */

    while (len >= 64) {
    memcpy(ctx.in, buf, 64);
    cMD5::byteReverse(ctx.in, 16);
    MD5Transform(ctx.buf, (unsigned long *) ctx.in);
    buf += 64;
    len -= 64;
    }

    /* Handle any remaining bytes of data. */

    memcpy(ctx.in, buf, len);
}


/*
 * Convert signature to CString
 */
char* cMD5::MD5FinalToString()
{
    unsigned char signature[16];
    MD5Final(signature);

    ms8_MD5[0] = 0;
    char s8_Temp[5];
    for (int i=0; i<16; i++) 
    {
        sprintf(s8_Temp, "%02X", signature[i]);
        strcat(ms8_MD5, s8_Temp);
    }

    return ms8_MD5;
}


/*
 * Final wrapup - pad to 64-byte boundary with the bit pattern 
 * 1 0* (64-bit count of bits processed, MSB-first)
 */
void cMD5::MD5Final(unsigned char digest[16])
{
    unsigned count;
    unsigned char *p;

    /* Compute number of bytes mod 64 */
    count = (ctx.bits[0] >> 3) & 0x3F;

    /* Set the first char of padding to 0x80.  This is safe since there is
       always at least one byte free */
    p = ctx.in + count;
    *p++ = 0x80;

    /* Bytes of padding needed to make 64 bytes */
    count = 64 - 1 - count;

    /* Pad out to 56 mod 64 */
    if (count < 8) {
    /* Two lots of padding:  Pad the first block to 64 bytes */
    memset(p, 0, count);
    cMD5::byteReverse(ctx.in, 16);
    MD5Transform(ctx.buf, (unsigned long *) ctx.in);

    /* Now fill the next block with 56 bytes */
    memset(ctx.in, 0, 56);
    } else {
    /* Pad block to 56 bytes */
    memset(p, 0, count - 8);
    }
    cMD5::byteReverse(ctx.in, 14);

    /* Append length in bits and transform */
    ((unsigned long *) ctx.in)[14] = ctx.bits[0];
    ((unsigned long *) ctx.in)[15] = ctx.bits[1];

    MD5Transform(ctx.buf, (unsigned long *) ctx.in);
    cMD5::byteReverse((unsigned char *) ctx.buf, 4);
    memcpy(digest, ctx.buf, 16);
    memset(&ctx, 0, sizeof(MD5Context));        /* In case it's sensitive */
}


/* The four core functions - F1 is optimized somewhat */

/* #define F1(x, y, z) (x & y | ~x & z) */
#define F1(x, y, z) (z ^ (x & (y ^ z)))
#define F2(x, y, z) F1(z, x, y)
#define F3(x, y, z) (x ^ y ^ z)
#define F4(x, y, z) (y ^ (x | ~z))

/* This is the central step in the MD5 algorithm. */
#define MD5STEP(f, w, x, y, z, data, s) \
    ( w += f(x, y, z) + data,  w = w<<s | w>>(32-s),  w += x )

/*
 * The core of the MD5 algorithm, this alters an existing MD5 hash to
 * reflect the addition of 16 longwords of new data.  MD5Update blocks
 * the data and converts bytes into longwords for this routine.
 */
void cMD5::MD5Transform(unsigned long buf[4], unsigned long in[16])
{
    register unsigned long a, b, c, d;

    a = buf[0];
    b = buf[1];
    c = buf[2];
    d = buf[3];

    MD5STEP(F1, a, b, c, d, in[0] + 0xd76aa478, 7);
    MD5STEP(F1, d, a, b, c, in[1] + 0xe8c7b756, 12);
    MD5STEP(F1, c, d, a, b, in[2] + 0x242070db, 17);
    MD5STEP(F1, b, c, d, a, in[3] + 0xc1bdceee, 22);
    MD5STEP(F1, a, b, c, d, in[4] + 0xf57c0faf, 7);
    MD5STEP(F1, d, a, b, c, in[5] + 0x4787c62a, 12);
    MD5STEP(F1, c, d, a, b, in[6] + 0xa8304613, 17);
    MD5STEP(F1, b, c, d, a, in[7] + 0xfd469501, 22);
    MD5STEP(F1, a, b, c, d, in[8] + 0x698098d8, 7);
    MD5STEP(F1, d, a, b, c, in[9] + 0x8b44f7af, 12);
    MD5STEP(F1, c, d, a, b, in[10] + 0xffff5bb1, 17);
    MD5STEP(F1, b, c, d, a, in[11] + 0x895cd7be, 22);
    MD5STEP(F1, a, b, c, d, in[12] + 0x6b901122, 7);
    MD5STEP(F1, d, a, b, c, in[13] + 0xfd987193, 12);
    MD5STEP(F1, c, d, a, b, in[14] + 0xa679438e, 17);
    MD5STEP(F1, b, c, d, a, in[15] + 0x49b40821, 22);

    MD5STEP(F2, a, b, c, d, in[1] + 0xf61e2562, 5);
    MD5STEP(F2, d, a, b, c, in[6] + 0xc040b340, 9);
    MD5STEP(F2, c, d, a, b, in[11] + 0x265e5a51, 14);
    MD5STEP(F2, b, c, d, a, in[0] + 0xe9b6c7aa, 20);
    MD5STEP(F2, a, b, c, d, in[5] + 0xd62f105d, 5);
    MD5STEP(F2, d, a, b, c, in[10] + 0x02441453, 9);
    MD5STEP(F2, c, d, a, b, in[15] + 0xd8a1e681, 14);
    MD5STEP(F2, b, c, d, a, in[4] + 0xe7d3fbc8, 20);
    MD5STEP(F2, a, b, c, d, in[9] + 0x21e1cde6, 5);
    MD5STEP(F2, d, a, b, c, in[14] + 0xc33707d6, 9);
    MD5STEP(F2, c, d, a, b, in[3] + 0xf4d50d87, 14);
    MD5STEP(F2, b, c, d, a, in[8] + 0x455a14ed, 20);
    MD5STEP(F2, a, b, c, d, in[13] + 0xa9e3e905, 5);
    MD5STEP(F2, d, a, b, c, in[2] + 0xfcefa3f8, 9);
    MD5STEP(F2, c, d, a, b, in[7] + 0x676f02d9, 14);
    MD5STEP(F2, b, c, d, a, in[12] + 0x8d2a4c8a, 20);

    MD5STEP(F3, a, b, c, d, in[5] + 0xfffa3942, 4);
    MD5STEP(F3, d, a, b, c, in[8] + 0x8771f681, 11);
    MD5STEP(F3, c, d, a, b, in[11] + 0x6d9d6122, 16);
    MD5STEP(F3, b, c, d, a, in[14] + 0xfde5380c, 23);
    MD5STEP(F3, a, b, c, d, in[1] + 0xa4beea44, 4);
    MD5STEP(F3, d, a, b, c, in[4] + 0x4bdecfa9, 11);
    MD5STEP(F3, c, d, a, b, in[7] + 0xf6bb4b60, 16);
    MD5STEP(F3, b, c, d, a, in[10] + 0xbebfbc70, 23);
    MD5STEP(F3, a, b, c, d, in[13] + 0x289b7ec6, 4);
    MD5STEP(F3, d, a, b, c, in[0] + 0xeaa127fa, 11);
    MD5STEP(F3, c, d, a, b, in[3] + 0xd4ef3085, 16);
    MD5STEP(F3, b, c, d, a, in[6] + 0x04881d05, 23);
    MD5STEP(F3, a, b, c, d, in[9] + 0xd9d4d039, 4);
    MD5STEP(F3, d, a, b, c, in[12] + 0xe6db99e5, 11);
    MD5STEP(F3, c, d, a, b, in[15] + 0x1fa27cf8, 16);
    MD5STEP(F3, b, c, d, a, in[2] + 0xc4ac5665, 23);

    MD5STEP(F4, a, b, c, d, in[0] + 0xf4292244, 6);
    MD5STEP(F4, d, a, b, c, in[7] + 0x432aff97, 10);
    MD5STEP(F4, c, d, a, b, in[14] + 0xab9423a7, 15);
    MD5STEP(F4, b, c, d, a, in[5] + 0xfc93a039, 21);
    MD5STEP(F4, a, b, c, d, in[12] + 0x655b59c3, 6);
    MD5STEP(F4, d, a, b, c, in[3] + 0x8f0ccc92, 10);
    MD5STEP(F4, c, d, a, b, in[10] + 0xffeff47d, 15);
    MD5STEP(F4, b, c, d, a, in[1] + 0x85845dd1, 21);
    MD5STEP(F4, a, b, c, d, in[8] + 0x6fa87e4f, 6);
    MD5STEP(F4, d, a, b, c, in[15] + 0xfe2ce6e0, 10);
    MD5STEP(F4, c, d, a, b, in[6] + 0xa3014314, 15);
    MD5STEP(F4, b, c, d, a, in[13] + 0x4e0811a1, 21);
    MD5STEP(F4, a, b, c, d, in[4] + 0xf7537e82, 6);
    MD5STEP(F4, d, a, b, c, in[11] + 0xbd3af235, 10);
    MD5STEP(F4, c, d, a, b, in[2] + 0x2ad7d2bb, 15);
    MD5STEP(F4, b, c, d, a, in[9] + 0xeb86d391, 21);

    buf[0] += a;
    buf[1] += b;
    buf[2] += c;
    buf[3] += d;
}

#ifdef sgi
#define HIGHFIRST
#endif

#ifdef sun
#define HIGHFIRST
#endif


#ifndef HIGHFIRST
    void cMD5::byteReverse(unsigned char *buf, unsigned longs)  
    {
        // Nothing
    }
#else
    // Note: this code is harmless on little-endian machines.
    void cMD5::byteReverse(unsigned char *buf, unsigned longs)
    {
        unsigned long t;
        do {
        t = (unsigned long) ((unsigned) buf[3] << 8 | buf[2]) << 16 |
            ((unsigned) buf[1] << 8 | buf[0]);
        *(unsigned long *) buf = t;
        buf += 4;
        } while (--longs);
    }
#endif