I know that this annoys people every day, and right now, I had to sit through an Episode of Stellvia with this annoying pop up coming up every 10 minutes, so I googled a solution, and first result showed me how to disable it.
Updating your computer is almost complete - Restart Now?
This should help those of you who hate being nagged to death to restart their computers.
Oh, Keep in mind that this WILL NOT work on Windows XP home edition. Don't ask me how to change it on that version of windows, cause I've never used it, I don't touch "Home" editions of software from Micro$ith.