Blue screen of death upon shut down

When I shut down my computer running vista it gets the part where it reads "shutting down..." and the the screen turns black for a second and I get the blue screen and the computer restarts. It usually turns on fine (except it bluescreened once a few days ago when I booted it on for the first time after it had been sitting off for a few days) and it runs great. I did a registry clean and it still happens. Any Ideas?