My computer behaves in a strange way when I tell it to shut down.

The hard drive shuts down but leaves the picture on screen.
The hard drive then starts up again for a few seconds and then shuts down
again with picture still on screen.
For a third time, the hard drive shuts down but at this point the whole
computer
turns of as I instructed it to do.

Worked fine under XP

Vista home premium
Gigabyte GA-M51GM-S2G motherboard
1 gig memory
Athlon 64 X2 Dual Core 3800+ 2.00 GHz

Thanks for any suggestions
Regards
Philip