[Mauzen]

The default windows 7 defragger is great and theres no real need to use an external one if you run win 7, MS really improved that compared to the shitty earlier ones. Files shouldnt be corrupted unless the hard disk is already damaged (and so some fragments are written to broken clusters), or you shutdown the computer while its still running, the currently buffered data then disappears in the black computer void with the most tools.
Also important to know: Do not defrag SSD drives at any chance. They wont get any slower even with high fragmentation, and moving around all the data for defragging just drastically their lifetime.