Vista performs quite well here on my P4-550 with 6600GT. Possibly it's because you upgraded? Although supported, I always do a fresh install. This guarantees there's no legacy driver or system file that makes the system run slower.
BTW, I had OSX86 on this box to test and it was much slower. Probably due to the fact that there weren't any good Nvidia drivers (I used the TITAN driver).
