Never used a free defragger, tho I'd try the best free defragger first....
JkDefrag.....
"J
kDefrag is a disk defragmenter and optimizer for Windows 2000/2003/XP/Vista/2008/X64. Completely automatic and very easy to use, fast, low overhead, with several optimization strategies, and can handle floppies, USB disks, memory sticks, and anything else that looks like a disk to Windows......"
....and....
JkDefragGUI [may not need this]
"....a Graphical User Interface which allows you to easily select all the features JkDefrag, a freeware defragmentation tool made by Jeroen Kessels has to offer".
....these below aren't exactly free, but there two of the best....
Diskeeper 2008 Pro .....used this one for an age .....flawless, stable, quite fast defrag (after 1st run).... worked great for me.
Perfectdisk 2008 .....currently testing / using this one ...seems very good also.