Hmm, in that case, its not what I was going to suggest, which was maybe the fact you weren't running the right video drivers (for example using the default VGA ones).
I'm sure I have had this type of problem before though, and this used to happen once every month or so for no reason what so ever.
I fixed it by running System Restore, and restoring to an earlier date, so if you have a recent back-up (won't delete anything), I would try that. Even going back a day fixed it for me, so have no idea what caused it.
Hope this helps though. Its worth a try