How about defragmenting the drive? Will that overwrite 'deleted' files?
No.
Actually, nothing short of shredding will truly destroy data before it's overwritten with new data. Even repartitioned drives can be "recovered" by companies that offer that service.
How do I know? I've recovered data for my employers and I've sent drives out to data recovery firms. If you want your data to be safe from snooping employers, keep it off their computers.
Chris