My PC keeps switching off & rebooting?

A desktop PC is NOT an hand held device. Windows XP just can't run on any hand held device, install on a desktop or laptop.

Seriously...
Check if it's overheating. If the CPU get to hot, the computer will shut down to prevent damaging itself.

Open the case and check the heat sink of the CPU. Use a can of compressed air to blow away the dust. Dust accumulation prevent an adequate air circulation and act as thermal insulation.

Make sure that the fan over the CPU's heat sink is working properly. Replace it if it don't work.

Electro
January 2013