restarting computer

Clarifications

because of the ntfs file system. You will need to run chkdsk drive: /f . If it comes up with errors that cannot be fixed close out and open explorer and right mouse click on C: select properties, tools, check now select Automatically fix file system errors. A message will come up and say that the disk check could not be performed, click yes. If that does not work download the Ultimate Boot CD. Boot the PC from the CD and run chkdsk drive: /f from the command prompt. It will ask to unmount the volume. let it. The cursor will blink on the bottom left of the screen when it is finished. Remove the CD and reboot.
Lets know how you go.

why a computer will reboot. If you're running Windows XP, you may have it configured to restart on error, rather than stop at the Blue Screen Of Death. Though these screens seem cryptic at times, they can be helpful in narrowing down a driver/hardware problem. Right click on My Computer, select Properties, click on the Advanced tab, click on the Startup and Recovery's 'Settings' box. In the middle of the next dialog box, make sure under the System Failure options that the 'Automatically Restart' option is unchecked. This way, if Windows IS detecting a problem, the system will stop at the BSOD and maybe show more detail if something is wrong. Have you checked the Event Viewer for any possible signs of Application Hangs? Any chance teh PC is overheating?

if there is any of your hardware that has the same irq then when you use them both at the same time then the computer will turn off right then. go to start then right click my computer go to properties and then the hardware tab chose the ddevice manager and if there are 2 or more pieces that have the same irq then change it to some thing else. this is most likly to be the promblem if you have added any new hardware latly. al so the inside of the compter could be very dirty causing the computer to over heat and turn off right then. and last the power supplly could be shot, you might have to buy a new one. i hope that this works for you

If you leave the computer on, but doing nothing, does the problem still occur?

If yes, if you boot to safe mode, and let it sit will it do it?
If yes again, then look to cpu overheating, power supply or power source as the culprit.

if no in safe mode but yes in normal mode, focus your attention on drivers and resident apps (antivirus, etc..) try disabling individual ones where possible (msconfig)and observe results.

if no to both (ie it only happens while actively using the system, booted in normal mode), can you identify a specific application (or group of applications) which is open when it happens?

Also, get memtest (www.memtest.org) and run it to confirm you don't have a memory problem which the bios checks are missing.

I had a computer doing that.

It was the power supply. It was rated "300 watts" but it fell out of DC regulation at about half that load. Get a voltmeter and measure it at one of the disk drive connectors. You should measure 12 V between yellow and black. 5 V between red and black. If the "12" is below about 11.8 or the "5" is less than 4.85, the supply is overloaded.

The computer could run at those voltages, but when they're low like that the supply isn't regulating. When the drive seeks or the CPU jumps from idle to run, the voltages droop a lot further than that for a few milliseconds. The disturbance is enough to bump the reset on the motherboard.

I never would have figured it out if I hadn't chased a similar problem in the lab years ago. I had a decent oscilloscope and probe back then and got a picture of the event.