I have a Dell Dimension 9100 running XP Home that is freezing fairly regularly. It boots and performs fine, but after a period of time ranging from a few minutes to 30 minutes it gets sluggish responding to events and then freezes completely. I ran a malware and AV scan and found nothing. Windows is up to date.
I then booted into safe mode to run a malware scan and it kicked a BSOD after about 8 minutes. The next time I went into safe mode it ran fine for more than 30 minutes.
Ran HijackThis which came back clean.
I've tested each stick of RAM individually and it freezes at some point for all. Running an AVG scan I could see the system slow down as it was scanning files and then if I clicked around on various windows after a few clicks the system alarm would start a continuous beep and it would lock up.
Finally, I used a CPU stress test utility to warm up the CPU. Doing this will hang the system within a minute every time.