PC lockup/freeze

I have a WinXP Pro system using an AMD 1.4 CPU, 512 ram, ATI Graphics, on-board sound, CD/RW, and a DVD burner. About 4 weeks ago it started to totally lockup. No response at all (not even the num or caps lock lights would change). I updated the graphics driver, didn’t work so I installed a new graphics board, same thing. I ran Norton A/V, nothing found. Changed RAM,changed power supplies, unplugged both optical drives, network, changed keyboards and mouse. Even starting in safe mode the PC will lock up. Sometimes it will work for 10 mins, sometimes it will work for 2+ hrs. When it locks, a re-set will work to get it re-started, and it will work again for the same time frame. Any ideas before I do a format and re-install?
Thanks