Often, the problem (especially with VISTA) is merely an out of date driver. Figgering out which one is a bit of work, but it boils down to Chipset, BIOS, Video Graphics, or Sound on most computers on which we have worked.
You might keep a few notes on what you are doing at the time of the Blue screen... both just before and at the time of the crash.
It can also be an infestation, so if the problem continues, run the latest version of HiJack This, and let the resident experts here take a look at it.
