I've got a laptop (HP 2307TX)which, when the AC Adapter is connected, will shutdown but 4/5 seconds later it will power on again! Without the AC Adapter (battery only) it can shutdown fine, but if you plug the AC adapter back in again it will power on by itself.
Have latest BIOS, Vista is fully patched (SP2+hotfixes). Booted into safe mode and then tested shutdown behaviour, same result. CMOS Battery is reading 3.1V (using a DMM).