PC rebooting on its own.

Right click my computer --> properties --> advanced --> startup / recovery --> uncheck automatically reboot...

Also, check the event logs to see if a device or bug is causing the reboot.

Another thing to do is run McAfees stinger program to ensure one of the latest virii hasn't infected the system.

http://download.nai.com/products/mcafee-avert/stinger.exe (remove any spaces provided by techrepublic)

If the pc reboots on printing, update the printer drivers.

Good luck

On the hardware side, I have seen this problem if the memory modules accumulate dust or are not seated properly. Try to remove the RAM modules, dust the slots, re-seat them. Do the same with other cards, and processor (only if u r comfortable with it).
Check for any slots broken or damaged, which are not able to 'seat' cards proerply etc.
See if the problem still persists.
Best of luck.

Are you connected to a network when it happens. Does it give you any forewarning when it reboots. The blaster virus and many of its spin-offs cause the RPC to restart your computer. Even if you are not infected but you connect to a computer that has been infected, though isn't anymore, it can still cause your computer to reboot. Try disconnecting from any network and see if the computer restarts on its own. If it does you need to get the Microsoft update and install it while your computer is disconnected from any network.

I recently had this same problem on my personal PC. As it turned out the power supply, mother board and 256mb ram were all fried. I had tried several of the software solutions and it seemed I would have it back for a day or 2 but eventually it would not boot at all. i ended up replacing the system.

continuous rebooting is a sign of insufficient power supply. Replace your power supply

This could possibly be because of some software / spware/adware files as well. Try too perform a cleanboot and check if the problem persists.

The following MSKB article helps in this,

http://support.microsoft.com/default.aspx?kbid=281770

I have had this happen with a processor was overheating as well. If your bios allows, you might monitor the CPU temp. If it seems to be running hot, check for thermal grease in between the processor and the heat sink and/or a fan that doesn't seem to be spinning as fast as it should (or if it isn't running at all).

Do you get an error message telling you that it will have to shut down and reboot? If you do it's a virus. Msblast is the name of it. Go to mcafee.com and look for it, they will tell you what to do.