Windows XP freezing randomly, only while playing games

"Is your new power supply rated lower that your previous one? What is the wattage? Note that the quality (and price) also affect a PSU's ability to deliver (it's never 100%). Above 400 watts should be fine though.

I had the same problem when running a 200watt GTX260 on a 430watt power supply (never had a problem with a 7900GT). I realize a 7600GT will not nearly take that much wattage (around 50watts in 3D), but it could be a sign that your PSU is borderline or poorly builded. My old PSU emittied a slight whining sound whenever it was in 3D mode, showing it was being put to the limit.

Try disabling any overclocking as well."