Defragment

if i'm not mistaken, defrag can not finish if other processes are running.
i don't think doing this damages the harddrive in any way...

You're not likely to hurt anything, but you're not likely to accomplish anything, either. Defrag should be the only application running. It will detect any change to information written to the drive being defragmented, typically causing defrag to stop and restart from the beginning. Generally, it is most efficient and effective to run defrag while in safe mode.