this is probably due to a fault with the graphics driver. It happened with me once... i had to reformat and reinstall the OS...
tell me one thing, if a black screen comes, u are not able to do anything?? Restarting is the only solution left with you?
Did u install vista on your own, or was it pre-installed?? if u did it on your own, then there may be a driver issue, if it was pre-installed, imo, the drivers should be up to date (hopefully).
Try starting in Safe Mode a few times, then to normal mode. Maybe that can do the trick...
In safe mode, remove all items from startup, coz some program may be conflicting with the starting up of windows...
Let me know if it works...