Hmm, a Windows re-install is usually the easiest method. My experience: My bro offered me the chance to try out an ATI Radeon AIW for a weekend. So I pop out my Voodoo5 after changing it to Standard VGA, pop in the Radeon, reboot, install Radeon drivers. Upon re-start of Windows I blue screen, re-start in Safe Mode. I go and delete every 3dfx/Voodoo key in the Registry, even some that I didn't need to(like some game generic vidcard entries), re-start, and re-install Radeon drivers. Re-start Windows, Blue screen, continued trying various things for 3-4hours(I really didn't want to re-install Windows). I gave up, ripped it out including it's drivers, threw my Voodoo5 back in. Re-start, Blue screened, ripped out my hair, decided to re-install Windows.
Doing a new Windows install will save you a lot of potential headache.