Are you logged in as Administrator or a user member of Administrators? Are you sure you are Admin? sorry have to be sure, no other have replied..
Do you have SP1 and DX9 installed? (SP1 has combatilbility updates I think)
The 1.02 patch? no-cd fix?
right-click a shortcut to the games .exe file, select run as and use administrator. Try create another account member of the administrator group only.
I've played it with and without the latest windows update files.
Ive also pointed all paths to install location in fallout2.cfg. I dont have to reinstall the game either if I reinstall XP (which is very convenient).
It does work at my PC so you have least a working reference.
my PC:
fallout 1.02 uk patch + no-cd fix
winXP pro SP1 DX8.1
user member of administrators
gf3 reference driver 40.41