function AuthenticateUser(UserName, Password, Domain)
dim strUser
' assume failure
AuthenticateUser = false
 
strUser = UserName
strPassword = Password
 
strQuery = "SELECT cn FROM 'LDAP://" & Domain & "' WHERE objectClass='*' "
set oConn = server.CreateObject("ADODB.Connection")
oConn.Provider = "ADsDSOOBJECT"
oConn.Properties("User ID") = strUser
oConn.Properties("Password") = strPassword
oConn.Properties("Encrypt Password") = true
oConn.open "DS Query", strUser, strPassword
 
set cmd = server.CreateObject("ADODB.Command")
set cmd.ActiveConnection = oConn
cmd.CommandText = strQuery
on error resume next
set oRS = cmd.Execute
if oRS.bof or oRS.eof then
AuthenticateUser = false
else
AuthenticateUser = true
end if
set oRS = nothing
set oConn = nothing
 
end function

public bool AuthenticateUser(string userName, string password, string domain)
        {
            bool authenticateUser = false;
 
            string strQuery = "SELECT cn FROM 'LDAP://" + domain + "' WHERE objectClass='*' ";
 
            ADODB.Connection connection = new  ADODB.Connection();
            connection.Open("Provider=ADsDSOOBJECT", userName, password, 0);
 
 
             ADODB.Command cmd = new  ADODB.Command { ActiveConnection = connection, CommandText = strQuery };
            object objRecAff;
            object parameters = cmd.Parameters;
            ADODB.Recordset oRS = cmd.Execute(out objRecAff, ref parameters, 0 );
            if (!oRS.BOF || !oRS.EOF)
            {
                authenticateUser = true;
            }
 
            oRS.Close();
            connection.Close();
 
            return authenticateUser;
        }