Windows XP has an option for auto rebooting upon system failure. Usually in the earlier versions of windows like win98/ME you would get a "bluescreen" saying that an error has ocurred and you would have to manually reboot your computer. This is what happens in XP but, they now reboot the computer for you right away. The "bluescreen" still appears but the computer will reboot so fast that you wont be able to see what it says, if you even notice it at all. You can, if you wish, remove this by unchecking the option by going to "system properties" (rightclick my computer and click properties is an easy way to do this) then look under the tab advanced, click the settings button for "start up and recovery". In the new window, looks for a checkbox in the section "failure" with the caption "automatically restart". Unchecking this will stop XP from automatically reboot your computer whenever your system fails allowing you too see the "bluescreen" along with the info on why your system failed.
This wont solve the problem to why your system have these failures but you might be able to figure out why, by reading the info provided on the "bluescreen" (it isnt very userfriendly and gives a whole lot of techie info that you would only understand would you be working with these kind of things, but its worth a shot).
If you cant solve the problem on your own, try posting the information you get from the bluescreen here and maybe someone can help you.
Hope this helps.
Good luck!