If you are diligent about keeping your machine running optimally, as I try to be, check out this screensaver which, when it runs, it defrags your hard drive. Don't be alarmed by all the comments you will get on it, but it will surely get someone's attention. Us this if you are constantly forgetting about defragging your drive.
http://www.kessels.com/JkDefrag/
Take the screensaver program and copy it over the system32 directory and then right click on your desktop > select screensaver and pick the JKDefrag screensaver and you're done.