Smart Defrag is specifically built for Windows XP and more specifically, people like me who forget or couldn't be bothered to Defrag there hard disk. The application once installed will wait until your CPU usage falls below 20% and automatically start defragging the hardisk. The program is surprisingly small and efficient, and unlike most defraggers you don't even know it's running. Vista in my opinion could learn a thing or two from Smart Defrag. The link to Smart Defrag is
http://www.iobit.com/iobitsmartdefrag.html