The disk defrag utility included in XP is painfully slow and the one in Vista is atrocious.  Sure sure they will do the job, but why wait around and deal with Windows' clunky utility when you can download Auslogics Disk Defrag for free.

This is my favorite utility to do the job.  I find it works blazing fast compared to Windows' utility.  Launch the application, select your drive, and you're ready to put those files in contiguous order.

The free version includes a fragment map and report upon successful completion so you know where certain files may be using a large chunk of your drive.