system freeze

I've had xp for a couple of years. I have been getting total lockup while logged on. Randomly freezes mouse on screen, keyboard dead. The smart-switch on/off mounted on the computer fails to function. I have to unplug and replug power supply to restart. Occurs after 10-15 min, up to several hours.
I've updated to present and run srvc pk 2. I've tried all sorts of registry cleaners, spyware removers, defragers, virus removers, etc.
I began building and teching my own computers in 83 but for the last few years I havn't been under the hood much.
Is Spider solitare suspect?
Intel Pentium III processor, Version: x86 Family 6, Model 7, Stepping 3, Speed: 451 MHz. 320Mb

 

Maybe heat? Motherboard Monitor will let you keep an eye on temps.

Johanna