>Is there a Windows API function that I can use to determine what Groups a User is a member of?
>
>I'm talking about the Users and Groups that are in the Windows NT User Manager.

The easiest mechanism is the ADSI COM components, which can allow you to get at nearly the full security model for NT and AD domains, as well as providing support for other platforms like NDS. See the ADSI/WMI sites in my sig block for more details.