Windows restarts without being promted to.

I have windows xp home edition. My problem is that when I use my pc. It turns off ( without being prompted to, No logging off or windows is shutting down) Then it restarts. A little while later it does the same thing. I have cleaned out the dust on the inside. I have used the defragment, I have done error checking or scandisk. I have scanned th hard drive for virusses. But I still have the same issue. I have even changed the power cable.
Help will really be appreciated.