windows xp computer turning off and restarting by itself

I had a problem and cured it my computer kept shutting down in the middle of doing something it took me some time to find it but got there I am running xp pro and after replacing all components including ram and updating all device drivers in windows xp the problem was still there it kept hanging and crashing and shutting down I luckily found it when i installed nero7 premium at work and the computer at work shut down and slowed down. I removed the incd program but kept the rest of nero on and cured the problem.