how to implement active directory with loginpage

You can google it, you will get lot of code for same.

For your ease, you can use below API

[DllImport("advapi32.dll", SetLastError = true)]
public static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword,
    int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

You need to use Enumeration to call API -

public enum Logon32Type
{
    LOGON32_LOGON_INTERACTIVE = 2,
    LOGON32_LOGON_NETWORK = 3,
    LOGON32_LOGON_BATCH = 4,
    LOGON32_LOGON_SERVICE = 5,
    LOGON32_LOGON_UNLOCK = 7,
    LOGON32_LOGON_NETWORK_CLEARTEXT = 8,
    LOGON32_LOGON_NEW_CREDENTIALS = 9
}

public enum Logon32Provider
{
     LOGON32_PROVIDER_DEFAULT = 0,
     LOGON32_PROVIDER_WINNT35 = 1,
     LOGON32_PROVIDER_WINNT40 = 2,
     LOGON32_PROVIDER_WINNT50 = 3
}

Final Call -

IntPtr tokenHandle = new IntPtr(0);

bool isAuthenticate = LogonUser(userName, domainName, password,
(long)Logon32Type.LOGON32_LOGON_NETWORK, (long)Logon32Provider.LOGON32_PROVIDER_WINNT40,
ref tokenHandle);

Reference :
You also need to know what version of OS installed, you will get this from below link.
http://support.microsoft.com/kb/304283[^]

-- Thanks Rushi