Sudden appearance of "It's now safe to turn off your computer"

My XP installation is suddenly not shutting down the computer completely

when I shut down Windows. I ends with the message "It's now safe to turn off

your computer" but I can't turn it off except by holding the power button in

for 5 seconds which surely can't be the right way to do it.

Somethings gone wrong because when I try to run Vista upgrade I get a

message saying the computer doesn't have suitable power mangement control

(or something similar and I should update my BIOS). The installation wont

continue from within windows.

But if I install Vista as a clean installation on the same computer

(different partition) I get no such error and installation completes so the

problem lies with XP.

Any ideas where I should start looking?