Will the system shut off in safemode? Have your removed everything from startup? What do your log files say is happening?
If these are no help:
If you have a second computer at your disposal, with an internet connection (run housecall from www.pccillin.com
), or anitvirus of some kind, you should pull out your HDD, hook it up to the second computer, and scan it. This may or may not help, but it will eliminate one posibility.
Secondly, if you have an extra HDD you could reload windows on it and test your system for the same "features". If the system loads windows just fine and runs ok on the new HDD, then you got software issues of some sort. If it fails to load windows, or does the same thing on the new load, then you got hardware issues.
If you don't have another HDD or system to test with, then you might run a program on ypour system like a HDD diag tool, or a memory tester to hard your system resources. See if it will shutdown after some time. If it runs for hours with no problems then you probably have good hardware, but something is goofing up your OS load.
Ok, all that i got for now...