Computer Freezes when playing games

[hnbmilk]

Hello everybody,
For about the last 2 or 3 months my computer has been freezing within the first 15 minutes of playing a game. Before this time everything was running smoothly. What i find strange is that it occurs with EVERY game (from tetris to fallout 3) and it occurs at around the same time. Before it freezes everything runs perfectly. To try to fix this problem I upgraded my PSU, added a new case with way better cooling, reinstalled windows on a new hard drive, and updated all drivers. I think the cooling helped a little bit because now it takes maybe 5 minutes longer to freeze but it is still occurring on a 100% consistent basis. When it freezes the screen stops at a single frame and the sound continues to play as if nothing happened. The keyboard and mouse stop responding and occasionally the screen turns black and displays that there is no signal". The graphics card fan seems to be working properly and nothing seems to be overheating (all temps are pretty low). Any body have any idea what i can try next?

By the way, my specs are:

Windows XP professional
Asus 775DUAL-VSTA motherboard
Intel Pentium 4 CPU 3.00 gHz
2gB RAM
Radeon X1650 series AGP 512 MB
580w PSU

Thank you very much for the help