Windows (Vista, 7, 8) has its own automatic scheduler for defragging drives and you shouldn't need some external program to do it for you. If you own an SSD you should double-check that the drive isn't being defragged by Windows to begin with, especially if it isn't the system drive. I believe any new disks that get added to your system after Windows is installed automatically get added to the automatic defrag scheduler.

And to add to Ugh's post - http://en.wikipedia.org/wiki/Wear_leveling - which is actually already mentioned in his link.