Vista - Vista Ultimate RTM won't shutdown or restart

I've just upgraded my HP Compaq Presario M2000 notebook from XP Pro SP2 to
the RTM of Vista Ultimate. Most things are running okay (except trying to set
my TV up through the S-Video socket, but that's another matter), but using
Start --> Shutdown or Start --> Restart doesn't seem to work, in both cases
Windows shuts down, the screen turns off and that's as far as it gets, the
power button, wi-fi button, keyboard and HDD lights all stay lit and the
laptop just sits there. If I've done a restart, the laptop doesn't.
Sometimes, when a software installation requires a restart, the restart
works, on other occasions it doesn't: if I remember correctly, a windows
update (that updated the synaptics touchpad driver) restart failed, but the
restart required by the installation of the ATI Catalyst Control Centre and
drivers for the ATI Mobile Radeon M200 onboard graphics for Vista RTM
succeeded.

Any ideas anyone?

Thanks...