I've tried installing SP1 for Vista twice already, and each time it is fruitless; The computer restarts after download the update for about 2 hours, and then goes into an infinitely long turn off-turn on cycle(believe me, I left the computer alone alone for a whole day, and it kept restarting itself).

Does anyone know how to stop this from happening? I heard it was something about a registry configuration or error, but I don't really know what to do.

What messages do you get when it turns on and off?
What make and model system do you have? (this may point to a problem with certain systems and SP1).