Hangs upon login

1.7 Ghz system running XP. Its a Gateway system I bought
on October of 2001. XP has all of the most current
updates. I haven't added any new software in months. The
other evening it suddenly locked up (it wasn't running any
programs other than the windows screensaver) and when I
rebooted, the system acted normal except that when it got
to the initial "Windows XP" screen showing that it was
loading, the system suddenly shut down and powered off.
I've checked all the power and cables and they're fine.

I've reinstall XP. It successfully installed and when it
self-rebooted, I got the same initial "Windows XP" window
and it then shut itself off. I also note that I can't seem
to access the BIOS upon login to try to start the system in
Safe mode.

I'm thinking it might be hardware related. Any thoughts?

Thanks.