I feel for ya. I had windows me on my computer then changed to win2k, the performance of all the test dropped alot. I was expecting the scores and fps to go down because 9x is faster but my 3dbenchmarks took about a 1500-2000 hit. Everything i do to correct this, doesnt help. I know the marks arent that important, but it sucks nevertheless. I also tried to tweak my geforce with almost everything i could find. Changing from high quality to fastest and back again, until I gave up and just defaulted everything again. No matter what i cut off or on on my video card, or in windows itself i still cant get even close to the scores i got with windows9x.
sorry i couldnt be any help, but if you find a way to fix your problem let me know.