Can  anyone tell me what causes my P.C. TO RESTART ON ITS OWN ???

Is your computer restarting when you click to shut it down or booting up by itself when it was off for a while or restarting while you were doing something?

Answers for two of the cases:
http://support.microsoft.com/kb/320299/en-us

For the case of booting by itself, when it was already off, there are numerous entries in most BIOS setups for waking a computer, such as resume by USB, resume by PS2,resume by LAN. These are usually disabled by default, but perhaps they have been enabled either accidentally or by someone else using the computer.

   If you are using your computer and it, for no reason at all, shuts off and then starts back up, the problem could be a failing power supply.  Sometimes when a power supply is bad, it will run normally for a while and then turn off, then when the computer restarts it will run normally again and then shut off, so on and so forth.  It may run normally for hours at a time.
   When my computer did this, I could not figure out what was wrong.  I finally broke down and took it to a computer repair shop.  I explained what the problem was.  He listened with no indication that he knew what the problem was.  After I left, he didn't even test anything but instead replaced the power supply and ran it all night.  I could have done that.