I am having the same problem. It started with a worm
It wont log on under admin either, it just shows desktop, and immediately logs off
I was able to re-install windows, but it installed in a different directory, and that is how I got to run viruscan, by the way, if you go to macafee, they have a daily file that you can download unzip, and copy to a cd, an other PC helps to do the download and CD buning; then run it on the PC in question, that is how I fownd the virus, but I haven't been able to clear up the shutting down issue
Let me know if you found a fix, if you don't mind.