Using CTRL-ALT-DEL is one way to help, yeah. The reason it restarts is because something's gone and changed some file on the disk, so the defragger is forced to restart again. Sometimes though, it's Windows itself doing that, so you're kind of stuck, and need to somehow defrag it while Windows isn't running, such as during boot time instead.
Defragging will give you noticable performance boosts if you happen to stick everything (all data) on a single drive, so basically mixing Windows, programs, temp files, and your own files and documents, and everything else. If you're clever with partitioning though and seperate everything, you should rarely have to defrag, if ever, to see a large performance boost. I never defrag my drives, partly because I don't need to because of this seperation, and partly because it'd take too long on such large drives anyway