There are a countless number of times that I have had people complain that their computer is running slow, and I ask them if they defragmented it lately. They stand their puzzled as if I just asked what the square root of 32137561 is. Once I have shown them how to do it I know deep down that it will never be done again unless it is done by me.
By default Vista will defragment the computer every Sunday at 4:00AM which is probably the time everyone uses the computer the least. I am not quite sure how it will handle itself if the computer is turned off at that time because scheduled tasks can do two different things in that situation: run when the computer is started or wait until the next scheduled time. Hopefully they will just have it run when the computer is started but I don’t think that will be the case.
Image taken from the Windows SuperSite.