I don't like Vistas defragmenter that runs by default daily. It causes incremental backups to be huge !!
I would turn it off so it does not run automatically, and when you want to defrag or see how things are doing (say every month or so ) use Vistas own derfragger from a command prompt. This will give you a detailed verbose readout of whats what.
To do this click the start orb and right click on "command prompt". Select run as administrator. type exactly, defrag c: -a-v and that will take a couple of minutes to generate a report. Type defrag c: -v-w to defragment all file sizes on the c volume.