Tim,
There was an article a couple of months ago in VB Journal about using API calls to validate users on an NT Domain. It uses a LogonUser() API function to test the typed in user name and password in your app against the NT Database. It also does this without exposing the password since it performs workstation to server validation via the network. You also get the benefit of checking groups, logon times, etc.

If I remember, I will look up the issue tonight. I'm sure the article and sample code is on their web site by now.