I would see if you have latest of the following:
Latest Bios for your motherboard--is usually a number that flashes upon booting.
Chipset drivers for motherboard
Direct x 9c
Go to nvidia and download the latest drivers for the video card.
Latest Windows updates (i'm assuming your xp--i hope)
Just be warned this might not fix your issues but worth doing and should be done anyway.
Newer video cards need more powerful power supplies than the older cards did also.