Windows has a built-in defragmenter, and while there is nothing wrong with it, it is lacking in a few areas. For example, it cannot defragment the swap file. Power Defragmenter
is a front-end developed by a third party to make Contig, a very powerful command-line defragmentation utility much easier to use for the average user. Contig is made by SysInternals, who have a long history of making solid defragmentation software. Instructions on how to get Power Defragmenter working with Contig are available at eXcessive Software's site, the makers of Power Defragmenter.