Get into your BIOS and check the temperature of your processor after your computer freezes. I was having this same problem, and it was heat related for me. It's a quick fix with a new processor fan (they're like $10).
So...
Next time your computer freezes up, restart it. Before it loads the Windows XP screen, hit the DEL (Delete) key a couple of times to get into your BIOS. It's usually DEL, but it could be something else. When your computer is starting up, pay attention to what it says and look for the "Hit <key> to enter setup." Hit that key to get into your BIOS. Your BIOS will have temperature information in it somewhere. Just look around until you find it. If memory serves me correctly, the Athlon processors should be running right around 100° F. If you're up at like 140 or 150, you'll know that heat is the problem. Like I said earlier, a new processor fan is an easy fix for that solution.