Boot,shutdown,re-boot loop...anyone hear of this?

Hi all,
My WinXP Home pc began to crash with this very odd symptom.
As I boot up I can see the POST get processed and get a black windows splash
screen.This blinks off,returns and then the pc shuts down and immediately
begins to reboot.
An IT guy from work thought this might be a result of the recently noted
jpg vulnerability which can affect any program using jpg's.
I have installed a new HDD and have a functioning pc again with all the
updates & SP-2.
Antivirus scans (AOL' McAfee & Windows) reports no virus activity.
I can read the original c: drive but still can't get a boot.No safe
mode,diag. mode,nothing.
Any help is appreciated.
Thank you.
Jay.