O&O Defrag is not freeware software, according to my sources...
Anyways, I know that Version 10 is compatible with XP.
Usually, most software of this type will run on any Windows OS that has a defragmenter tool.
Windows has its own built in defragmenter
click Start> Programs> Accessories> System Tools> "Defgragmenter"
Auslogics is a great defragmenter. Check it out:
http://www.auslogics.com/disk-defrag/index.phpJkDefrag is also great, but does not look good:
http://www.kessels.com/JkDefrag/ (but looks are only aesthetics)