PC hardware: PC shutdowns itself and restarts after a while

PC shutdowns itself and restarts after a while

Dear All,

I have a windows xp home and p4 3 ghertz, 512 mb ram and 256 mg geforce6600 and for about last 1 year I had no problem but yesterday I installed DX update and the elder scrolls 4 : oblivion and tried to install several official plugins for the game but as I tried to install the plugins ( not the game, the game installed succesfully) my computer froze and I had to restart, to solve this I emptied all the files in Windows temp folder and restarted without startup programs and then installed the plugins sucessfully without any freeze..But as I started the game the computer shut down itself and restarted after about 10 minutes again itself. ( I had to restart the windows about ten times in 1 hour before the succesfully plugin installment )

Since yesterday the pc shuts down itself after opening but in changing time intervals from 10 minutes to 1 hour..After shut down the pc again restarts itself after a while, Possible cause is overheating but I wonder If the game itself or deleting the whole windows temp folder could cause such a problem.

After the computer shuts down, I cannot restart it on my own, I must wait to restart itself.

Note : Zone alarm ve AVG anti virus were installed, I uninstalled them for they could be a reason..
Best regards.