hi there welcome to dal the online computer help forum
Windows Xp has a feature whereby the operating system will cause the computer to reboot whenever a critical error is detected. If your Windows XP computer is rebooting, perhaps randomly and quite frequently then you could try turning this feature off which will instead give you an error message. Once you have the error message it should help either you or one of the tech's on our forum help you
How to turn off Auto Reboot in XP
Right click on My Computer
Select Properties
Select the Advanced Tab->Startup & Recovery section
Select the Settings button
uncheck "Automatically restart" Apply".
This will give you a blue screen withthe error message rather than rebooting the computer.
hope this helps