Look at your even viewer to see whats going on. XP is very similiar to 2000, I have and run both. To do this to see if we can help you any further. First of all disable the re-boot at errors problem. This may resolve your re-booting issue. All these steps will be on my win2k computer, so look around your XP setup.
Right click my computer
go to the advanced tab
start up and recovery (on the last button towards the bottom)
In the System Failure area
Uncheck --Automatically Reboot.
Now go to your Event viewer and post the errors you get.
Event viewer is located on
****Again this was done on a Windows 2000 Professional operating system, I do not take any resposiblity yadda yadda yadda****