If you do a lot of read/writes to a drive, usually the one with the swap file (virtual memory), you should defrag regularly. If you uninstall programs or delete large files, it pays to defrag so that any future writes will create contiguous files instead of the files being written to any free space on the drive. It's usually faster to access a contiguous file instead of a fragmented file. When defragging, try to use the consolidate function of the application.