My personal computer would sometimes restart (reboot) immediately after I have shut it down. Other than that, there doesn't seem to be any trouble with its functions.

Need more details about PC (age, CPU, OS) to help. Have you done regular maintenance (defrag etc.)?

My PC is about 3 to 4 years old, running windows xp. CPU is pentium 4, 256 mb memory.

256M of RAM is very light for Win XP. Since RAM is pretty cheap I would bump it up. What about regular maintenance?

Since this happens occasionally, it *might* be because XP thinks its detected a system failure and is set to automatic restart.

I'm not sure if it set by default, but you can check for this.

Press Windows key + Pause/break key together to bring up the system properties windows.

Go to to the advanced tab, Startup and Recovery settings, and check if the box (Automatic restart) is checked. If it is, uncheck it.

I know one of our PCs used to do this, but I can't remember how it was fixed, let me ask around.

Hello, If you have not already completed Spyware, Malware, virus-worm scans, that is where I would begin. Spybot Search and Destroy is free and effective. Adaware by Lavasoft is excellent and also free. Microsoft clean up is good as are various products by Trend Micro (also all free) You would be amazed at how much crap can appear on a system that seems to be free of all this. If not a software issue then move onto drivers. Video drivers, in particular, when corrupted can cause all kinds of shut down and reboot problems. Uninstall and reinstall them and see if that helps. If none of this works, let me know. RSM

Try starting in safe mode and see if it reboots. Go the event viewer logs and see if there is any indication of what the computer is doing just before reboot. Go to msconfig and turn off 1/2 start up items; if that does not fix, turn off the other 1/2 and see if there is something running in the background.