If you don't want to spend any money and you still aren't satisfied with Windows' built-in options, this thread on Lifehacker has some good suggestions. To begin with, DirMS is a free app that comes in GUI and command line versions. The GUI is, to put it mildly, austere, but it works. At the same web site you'll find an app called Buzzsaw that defragments "on-the-fly," i.e. it detects file fragmentation as it happens and undoes it when you've got spare processor cycles. Then there's Sysinternals' Contig, which is free and command line only, but if you need a GUI for it there's a free front-end for it called Power Defragmenter from eXcessive Software.