Want your OS to run quickly? Take a look at Piriform's Defraggler. If you are using Windows XP, you do not want to be defragmenting with its native defragmenter since it is extremely low on features, is way too slow, and doesn't do a good job at all. Defraggler is alot more versatile. If you want a defragmenter that shows you exactly which files are fragmented, how many fragments there are, and select which files you want to defragment, then I absolutely recommend this piece of software. It's very sleek, easy to understand and use, and the best thing about it? It's free! And is updated frequently by giving you a reminder that there's a new version.
For a full list of features, visit http://www.piriform.com/defraggler. Works on all NTFS and FAT32 systems, including Windows 7.