Well, if you've changed video card (and drivers) only, your problem is because video card or divers. No other reasons. If your previous video card was GeForce2/3, it's video card definitely. Try to set STABLE drivers from manufacturer of video card (i.e. ASUS, MicroStar, etc.), then if it will not help - try latest stable drivers from
http://www.nvidia.com. If it does not help - try to replace this video card by another GF4MX-420. Probably your new card has some defects. It happens.
However, I can guess 2 other reasons, but they are quite improbable:
1) Your video card is too hot and require additional cooling, so now your PC is overheating. Try to work with opened case.
2) New video card consumes a lot of power and your power unit cannot supply it.
Anyway the best you can do is to test GF4MX-420 which is known to be stable.