Posted 27 December 2010 - 09:05 AM
I wish there was a clear answer to your question, but as my best guess, it may be to do with shadow copies and how the Vista defragmenter has trouble with those. To get your space back, deleting all the restore points by resetting system restore (turn it off, reboot, and turn it back on) may help.
For the future, you can also try to defrag with a 'smarter' shadow copy-aware third party automatic defragger; it may minimize the loss of space due to shadow copy activity.
I am not 100% certain about all of the above, hence the liberal use of 'may' instead of 'will/is'.