Defragging Your Hard Drive under Vista

There's a good brief discussion of what happens to your hard disk to cause fragmentation of files and gradually slowing operation on the Error Forum

One of the things I appreciate about Windows Vista is the deframenting method, which, instead of locking up your computer for extended times when you ran it under XP or 2000, now runs automatically in moments when you aren't using your PC.

The article above also describes how to change the default settings in the Vista defreg program.