not all files being defragged

What would cause the xp defragger to not defrag every file, even if the files are 7kb and not being used at all? I never understood why it won't defrag every file that is not currently in use.

 

Windows (or any other defragger for that matter) cannot defrag files which have been locked such as opperating system files and some files by applications and the page file (during system in use) and the MFT zone.
Defragging doesnt just put the files back together in one string it also moves them to a new location on the hard drive and as a result is not able to do this if the file is in use or is locked for some reason (usually a protected file to prevent being tampered with by viruses) you can try this by un buffering media player and try moving a video or MP3 currently being played. It wont let you.

 

You can defrag some of the files the Windows defragger misses with Sysinternals PageDefrag, which defrags them on boot: http://majorgeeks.com/Pagedefrag_d603.html