Steve Thackery

I'm inclined to think this is a coincidence. I don't think there is any way
defragging your disk can result in a loss of disk space (not permanent,
anyway).

As the other contributors have said, Vista runs other activities which
consume disk space. I would take their advice re backups, Disk Cleanup,
etc.

Two last things to say about defragging. Firstly, manually defragging is
not a sensible response to seeing your disk space go down. Defragging is
not intended to release disk space, nor consume it. It simply reorganises
the data on the disk to make it faster to access. (In reality defragging
can release some space, but only a bit, and that isn't its real purpose).

Secondly, you should never need to defrag manually anyway. Vista kicks off
a defrag once a week automatically, and all the informed opinion is that it
works fine if you leave it to its own devices).

I would look elsewhere for what is using up your disk space. It is almost
certainly Vista doing its normal business, though, and I honestly think you
could safely ignore it.

SteveT