While Defraggler worked fine for me in XP, with Win 7, I had nothing but trouble with it. It would take forever to do the job and actually corrupted one of my data drives, which then corrupted one of my backup drives; I had to reformat the drive and restore the data from a second backup drive, the rebackup to the first data drive (after reformatting it) to recover from the crash. I ditched Defraggler after that.
I've found Win 7's native defragger works just fine. I have it set to automatically run during the wee hours once a week (my computer runs 24/7).