MY pc (3ghz P4,msi motherboard,80gig hdd,9500GT 1gig,windows 7 ultimate)
goes off (shuts down) automatically like disconnecting the power. Every time I play games its switches off. I have installed the newest drivers from nvidia's site for windows 7. Still the problem persists.
The 9500gt is a pretty low power card but it sounds like when you try to game it's asking for more power than the PSU can handle. That you are using a P4 says your system is old and PSUs tend to degrade over time. So if I had to guess I'd say it has reached the point where it can no longer power the card properly.
You could replace the PSU but this may be a good excuse to build a new system. For under $500 you can build a system several times better than the one you have now.
sorry, have a 500watt psu, figured it out in my own...had to run the games in compatibility mode in windows 7. The games are running smoothly again..so stupid windows 7(or nvidia) have to get the drivers upfated.