Access Denied Error Using GetObject("WinNT://"

I use the following code to check if a user is an admin in the login page of our intranet.

Set User = GetObject("WinNT://" & UserDomain & "/" & UserName & ",user"
For Each Group in User.Group
Select Case Group.Nam
Case "Domain Admins
Session("Admin")=Tru
Case "Domain Users
Session("User")=Tru
End Selec
Nex

The code works fine on IIS5 and Windows 2000. I have set up a test server with Windows 2003 and IIS6 and created the same application, however the second line of this code returns an error message

Active Directory Error '80070005
General access denied erro

It's obvioulsy a permission issue but I cannot figure out how to get around it.