in addition to the other posters, I had a similiar experience, however my system was very much different from yours therefore, this may help and then again it may not.

My problem was my video card. It seemed I had a junk card! I popped it in, and it worked for about an hour very well, then it just randomly would lock my PC up. I thought it was overheating so I bought a new dragon orb fan, and tried it out with the same results. I then installed it in another PC, and the got the same results.

My suggestion, update the drivers on your Geforce card first! You can get them at http://www.nvidia.com

Second, replace your existing card with a cheaper one, or one from a friend. Or install your current card in another system. See if you get the same results. If you do you will know its the card.

Also, make sure you got all the windows updates installed as well! Just open internet explorer, click on the tools tab, and then the windows update, follow the instructions.

If that still does not help,
Try a reinstall of windows XP, see if that clarifies matters.

Now on to hardware. Try uninstalling all the non essential components, like USB devices, printers, sound card, extra CDROMS/DVD/etc...
Do it one by one until you get it where it stops hanging on you.

IF that doesnt work, try a motherboard BIOS (basic input/output system) Just do a search on your MOTHERBOARD's site.
Another thing that might be hanging your system is your internet connection. Try running your machine without being connected for a day or so...
See if that corrects the problem, if so consult with your ISP about this issue and see if they can offer an updated driver or patch...

If none of this helps, and you still have this problem, post again and we shall see what we can come up with!
BB