Hayati Güney dilim döndüğünce ;)

using System.DirectoryServices;

    //srvr = ldap server, e.g. LDAP://domain.com
    //usr = user name
    //pwd = user password
    public bool IsAuthenticated(string srvr, string usr, string pwd)
    {
        bool authenticated = false;

        try
        {
            DirectoryEntry entry = new DirectoryEntry(srvr, usr, pwd);
            object nativeObject = entry.NativeObject;
            authenticated = true;
        }
        catch (DirectoryServicesCOMException cex)
        {
            //not authenticated; reason why is in cex
        }
        catch (Exception ex)
        {
            //not authenticated due to some other exception [this is optional]
        }
        return authenticated;
    }

Kaynak : http://stackoverflow.com/questions/290548/c-validate-a-username-and-password-against-active-directory