I played with this technique, but it falls apart if you change the NIC. Once the "MAC" part of the GUID is put in the Registry, it never changes. Conversly, if you know the location in the Registry that this value is stored, a user can change it, and it won't have anything to do with the real MAC. (I don't remember where in the registry I found it, but if it's important I could probably dig up the information.)

Rick