Low_B

Hello Supertenny ...
like Cocacola87 said check your CPU's temperature @ BIOS Setup and check your "Shutdown Temp Limit" maybe the limit was set too low.  Normal CPU temp is around 40-50C; Shutdown limit should be at 70/75C, also you can check your CPU fan's rpm, see if it is running too low.
Stay at that menu for 30 mins to see if your computer will shutdown or not.
If your CPU's temp is fine, that mean the shut down is cause by other hardware or software.
Try to boot up you computer with DOS instead of Window XP, you can download DOS 6.22 from internet. Run some dos command such as CHKDSK or SCANDISK ...
If your computer didn't shutdown when is running in DOS ... that mean is the Window XP cause the shutdown.