This will be a quick tutorial. I wish to explain why Windows 2000 and XP machine reboot alone. Whenever you ask a tech why your machine reboots by itself, most tech will automatically think of a hardware problem. Under Windows 95/98, it's true. Under Windows NT, 2000 and XP, it's false. Those OS come with an option that most people forget. If you go under System Properties in Control Panel, under the Advanced Section, under Start and Recovery Setting, you have an option that is turn on by default on Windows 2000 and XP name "Automatically Restart".This option enable to OS to reboot by itself whenever a Blue Screen of Death happen. This lead to a lot of tech to blame hardware for a problem that under Windows!! If the option "Write an event to the system Log" is on, by default it's on, Windows will log the event. After the reboot, you can check the event under the Source "Save Dump". A Blue Screen of Death will a lot created a memory dump. Check this thread to know more about memory dump.

In the event viewer, you'll see the error code for your Blue Screen of Death. Most of the time, Windows 2000 and XP will crash because of bad memory or bad drivers. My recommendation is to turn off the option "Automatically Restart" option on all machines you run to save you a lot of trouble later unless you have a very good memory to remember this tutorial.