Whenever I re-install on 98se I always:
1. Un-install thru "Add/remove programs", DON'T REBOOT!
2. Run DetDestroyer, un-install, DON'T REBOOT! (Thanks to whoever wrote that, GREAT utility!
3. Open up Windows Explorer and goto the C:\windows\inf\Nvagp.inf (or some drivers call it "Nvaml.inf", whatever starts with "Nva") then goto C:\windows\inf\other and delete NVIDIANVAGP.INF (or whatever is named "Nvidia" in there, if there's more than one delete all of them...but only the nvidia ones!) While you've got explorer open, go ahead and delete your entire C:\Program Files\RivaTuner directory. (You gotta re-install it each time clean.)
4. Goto your start menu, goto 'Run', type in "msconfig" and hit return. Goto the start-up tab and uncheck RivaTuner, DON'T REBOOT!
5. Goto the run box again and type in "regedit". BE CAREFUL IN THERE AS SCREWING UP WILL HOSE YOU'RE BOX!
Under "HKEY_CURRENT_USER\Software\NVIDIA Corporation" highlight the "NVIDIA Corporation" bit and hit the delete key. Confirm when asked. Then goto "HKEY_LOCAL_MACHINE\Software\NVIDIA Corporation" and delete it also. In that same directory there will be a branch called "Unwinder", highlight and delete that as well. ("HKEY_LOCAL_MACHINE\Software\Unwinder", Unwinder is the author of RivaTuner and that's where all the RivaTuner settings are kept.
6. Reboot. (FINALLY!)
7. Let it auto-search for a driver on reboot, if it finds the default adapter you've got a clean bill of health. Follow the prompts to install & reboot.
8. Install new drivers, re-install RivaTuner RC11.1
Ta-da! Simple, eh? With a little practice it takes less than 3 minutes. (My personal record so far is about 2...