Password Storage: How to do it.

I think it has error, for example: A user is:

string userId = "OriginalGriff";
            string password = "NotMyPassword";

And other user is:

string userId = "Original";
            string password = "GriffNotMyPassword";

Problem to store two user. My idea is:

private static byte[] GetSHA1(string userID, string password)
            {
            SHA1CryptoServiceProvider sha = new SHA1CryptoServiceProvider();
            byte[] hashedPassword = sha.ComputeHash(System.Text.Encoding.ASCII.GetBytes(userID);
            hashedPassword += sha.ComputeHash(System.Text.Encoding.ASCII.GetBytes(password);
            return sha.ComputeHash(hashedPassword);
            }

Thanks.