IsUserAnAdmin is a simple call and seems completely reliable for use in VB6 programs. It seems to work just fine even in Win8.
Please note that it reports whether the process works with administrator rights, and whether the user is in the Administrators group.
Having declared it this way, it should fully work in Windows 2000:
Private Declare Function IsUserAnAdmin Lib "shell32" Alias "#680" () As Long
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