Computer keeps restarting without warning

Your clients or even your own computer may keep restarting for no reason without warning. 
First of all, when a computer keeps rebooting it can be caused by a wide range of problems,
 from incompatable software such as Firewalls and Antivirus software, damaged or overheating
 parts inside your computer or even incompatable peripherals such as printers and scanners.
 Here are some computer repair tips to help you diagnose the problem.When a reboot occurs
 from within Windows XP, your computer will totally restart without telling you what caused it.
 This is the default behavior when Windows XP encounters a system failure.

To see what the error was you can change the default behavior by right clicking on 
“My Computer” and choosing “Properties”. Goto the “Advanced” Tab and under
 “Startup and Recovery” press the “Settings” button.

Under the “System Failure” box there will be a checkbox saying “Automatically Restart”. 
Untick this box and click Ok.

You can also check to see why your computer had a failure by checking the Events
 log for error messages. You can see the Event Viewer by going to Run and type “eventvwr”.

Another cause for random reboots is that the computers processor is getting too hot. To check this,
 enter the computers bios during boot (usually by pressing Delete, F1 or F10 when the 
computer just starts) and there will be an option about the computers temperature
 (often under “PC Health”).This should show you the current temperature of the computer
and the emergency shutdown temperature. The emergency shutdown temperature is a safeguard
 for when your computer gets too hot it will shut itself down to prevent damage.

You can either raise the shutdown temperature or switch it off completely, however be warned
 that you may damage your computer (or the clients) if the computer gets too hot. If the temperature 
was causing the reboots then you should consider getting better cooling for the computer such as
 more powerful CPU fans and more case fans.