Contrig is a command line utility (from Systernals) that works with NT4-2k-XP defrag. Power Defragmenter GUI adds a gui to you have a nice wizard to defrag files, folders, or entire drives.
Power Defragmenter GUI
http://www.softpedia.com/get/System/...agmenter.shtml
Contrig
http://www.sysinternals.com/Utilities/Contig.html
It runs extremly fast and defraged C: while it was running (note: it does ignore locked / in use files).
In one of the tests I did, I just downloaded a file via Bittorrent (very fragmented), and it's was actively sharing the completed file back out, it defragged the file while it was sending data from it. I then did Hash check and the file still the same hash value (file is in tact) after it was defragmented.
Free Defrag Alternative
EternalTempest
Hockster
Sysinternal is one of the most respected software companies by IT professionals. They put some great utilities.