Computer Freezes

Let me apologize in advance if I have posted this in the wrong forum, and if I am providing insufficient information. However, I am at my wit's ends! For a while now my computer seems to inexplicably freeze for no good reason, and I'm beginning to think it may be a video card or video driver problem. Here's what I've noticed:
- The problem doesn't seem to occur when I'm in safe mode
- It seems to happen most often when I'm actively online (i.e. - downloading or on the net)
- The computer is totally unresponsive (including Ctl+Alt+Del), but the mouse still moves around, the animation on the mouse is active, and the NUM LOCK key on the keyboard works.

I have reinstalled Windows XP Pro a couple of times, but the problem persists. I have an ATI Saphire (RADEON) X300SE video card, an AMD Athlon 64 bit 3000+ (1.8 GH) CPU and 1 Gig of DDR Ram.

Can anyone offer any suggestions? Thanks!