Computer Restarting by itself

I have read all i can on this but for my system it restarts on itself with no warning at all i got no error no blue screen and i have no spyware or virus problems and i have gone to the Startup and recovery setting and took off the autorestart and nothing it still does it i have replaced G cards RAM and 3 Power Supplys and i have cleaned all fans in my computer and it still restarts on it self i have reformated my drives from FAT32 to NTFS and reinstalled Xp Pro still nothing

here are my specs

AMD Althon 1500
1 GB DDR RAM
Gforce 5700 256 mb G card
XP PRO

 

This can be caused by overheating.. Tampered with your CPU lately?

 

it isnt over heating i got external fans on it and its always like 57degress with MBM

 

Could possibly be capacitors on the mobo going bad. If you've got some age on the system, look at the tops of some of them. If they are starting to bulge up or discolor, that might be causing random restarts.

 

Yep, sounds like a mobo fixin to go out on ya. I had a similar problem, and tried everything. Turned out to be a bad mobo. Might be something even as simple as a bad power contact. Check for scorch marks on the power pins on your mobo. As well as the capacitors previously mentioned.