I should've mentioned that you may need to manually install the drivers, simply running the .exe installer may install the driver without enabling it. Try extracting it first, then go to control panel, under Device Manager select Nvidia, Driver, Update Driver, Browse my computer for driver software, Let me pick from a list.., and then click on Have Disk and browse to the folder where you extracted the driver.
To make sure you've successfully installed a good driver, try refreshing the Windows Experience Index, you can find that under Control Pane >> System. If it gives a low number like 1 or 2 for graphics then you're probably still running the default drivers, if it's a higher number like say 4 or 5 then you should be good.
by the way i'm assuming you're using Vista, if you're on XP then the steps should work except for the Windows Experience Index part.
