[DllImport("ADVAPI32.DLL")]
        public static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword,
        int dwLogonType, int dwLogonProvider, out int phToken);
 
    const int LOGON32_LOGON_INTERACTIVE = 2;
        const int LOGON32_LOGON_NETWORK = 3;
        const int LOGON32_PROVIDER_DEFAULT = 0;
        const int LOGON32_PROVIDER_WINNT50 = 3;
        const int LOGON32_PROVIDER_WINNT40 = 2;
        const int LOGON32_PROVIDER_WINNT35 = 1; 

private bool isValidUser(string userName, string domainName, string passsWord)
        {
            bool Validateuser; int token1;
            Validateuser = LogonUser(userName, domainName, passsWord, LOGON32_LOGON_NETWORK, 0, out token1);

            if (Validateuser)
            {
                return true;
            }
            else
            {
                return false;
            }
        }