I had a similar problem when I upgraded from windows 98 (fat file system) to windows 2000 (nt file system). Back when autocad 2000 was put out they offered it for 2 windows platforms (fat or nt file systems), your copy could be for the fat file system and not nt. IIRC, Xp is based on the nt system.
In windows 2000 there was a way of telling the system to run the program in win98 mode. I'm not sure if xp can do that or not.