pavilion g3100uk

My mother in law recently bought a new HP system whcih has vista pre-
loaded on it. The PC is a reasonable spec but it wont shut down
properly. When she clicks start > shutdown > ok. the system goes
through all the Windows is logging off/ shutting down screens and then
the screen goes black but there power button is still lit on the base
unit and still has power going to it, (hold down power button for 5
secs turns it off).

Has anyone come across this before and if so please tell me what the
fix is?

the problem seems to be happening when Windows relinquishes control
back to the hardware/bios. Either way I've checked power management
and bios settings but cant find anything obvious.

Thanks in advance.

C.