Hardware: is an IBM Thinkcentre desktop with Windows XP Professional.
Question: System has started to shut down involuntarily and most times reboots, but not always. Why does it do this? How can I prevent it?
Try removing add-ons you have within your browswer such as your google tool bar.
It sound a lot like a hardware problem. Likely overheating. Could be the motherboard short circuiting. Do you hear your fan spin up before it turns off?
Nothing shows up on the screen, it just shuts down. When it reboots, it often resumes the same session. I cloned a new drive from another similar spare system so now I've experienced this with two different drives using the same motherboard.
Can you give us any more clues? Anything showing up on the screen?
Sounds like an overheating problem....make sure the ports are not blocked,,, and get a can of air and blow it all out.....