Active Directory/LDAP :: LDAP Authenticate AD Fail On IIS (SSL), But Successful In VS2008?

I am a newbie on a LDAP, now we want to re-deploy a web application that will validate user name and password with AD in a login.aspx

I had success to check the user and password in VS 2008, but when I publish to server it don't work, (they can login even wrong username and password) I had enable the anonymous access and intergrated windows authentication on IIS. we using authentication mode = windows in our web.config

using something like the following to check the AD account

System.DirectoryServices.DirectoryEntry entry = null;
bool isOpened = false;
bool isSuccess = false;
try
{
string dom = System.Configuration.ConfigurationManager.AppSettings["domain"];
entry =
new System.DirectoryServices.DirectoryEntry("WinNT://" + dom, tb_username.Text.Trim(), tb_password.Text.Trim());
isOpened = true;
Object obj = entry.NativeObject;

but still no luck on IIS server. Would you mind to give me a hand to solve this issue, I had google for a week and still can't solve :(