What kind of motherboard? give the hardware details. Restarts again and again could be a virus, a bad motherboard, or an error in
the security program (like McAfee).
Do the repair install of XP. The directions are posted at he top of this forum.
This does not erase your personal files, but does restore the OS files.
Never use a free antivirus from the internet. Use a pro antivirus, install, activate, update antivirus, and scan the entire computer.
Some viruses will prevent the repair install, or reformatting of the drive. You might have to bulk erase the current drive, or install a new drive, and start fresh.
You may recover your old files off the damaged drive by installing it as a second drive on a working computer. Then drag your files off the drive, on to a DVD or thumb drive. OR you can have a computer shop do it for you.