That's because in Windows you can either have the computer display a Blue screen or auto reboot if there is an OS error. That's what I was trying to get at, sorry I guess I didnt explain it well enough.
Anyway, here is how to do it in WinXP (which I guess you are using). Next time your computer WONT restart, it will display a blue screen. Take this info from the blue screen, write it down and post it here and we will be able to help you better <img src="/~stretch/ubbthreads/images/graemlins/smile.gif" alt="" />
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 with the error message rather than rebooting the computer.
If you need more explanation just pm me
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.