I was wondering if you had power management in your bios. If you do. It might be shutting down from there(not doing a windows shut down, just turning the power straight to off.)
Also download aida32 run it, this will tell you the type of mobo you have as well all other revlevent info in your system.
dfi nfII ultr infinity/asus p4s800
amd 2500xp m/p4 2.6
ati 9600/ati 9600 pro
512mb pc3200/512mb pc3200