A dust and lint layer on some cpu or other heatsink. An aging power supply or the infamous bad motherboard capacitors.

Then again... a classic. "Bad RAM."

There is not enough clues here to do anymore but guess.

-> Software? How about this one. You wrote XP. That one gets a WORM on install if you connect to the internet. Tell what you do to stop that. Hint: It's not an antivirus.

Bob