The first part of the defrag process involves the PC looking at all the files, working out where to best relocate them to defrag, and then working out the order to put them on the drive for maximum efficiency. If there are a lot of files on the disk, this will take a long time, and there will be no apparent action on the display, making it look like the process has hanged. Defrag is best done when you've got a few hours away from the computer (whilst watching a movie, or the TV, etc).