Its pretty severe when a unit just powers off with no warning. (usualy down to overheat) are you sure its not turn off and reboot (auto restart on error)
Navigate to the Control Panel in Windows XP by left-clicking on Start, followed by Settings and then choosing Control Panel.
In the Control Panel window, open System.
Note: In Microsoft Windows XP, depending on how your operating system is setup, you may not see the System icon. To correct this, click on the link on the left-hand side of the Control Panel window that says Switch to Classic View.
In the System Properties window, click on the Advanced tab.
Locate the Startup and Recovery area and click on the Settings button.
In the Startup and Recovery window, locate and uncheck the check box next to Automatically restart.
Click OK in the Startup and Recovery window.
Click OK in the System Properties window.
note what the BSOD is saying should be a STOP:0x