O+O defrag is what I use,it's quicker than the Windows defrag_and can run WITHOUT shutting down processes. You don't want to bit torrent,burn a CD,but can web surf and do light load stuff while its working. I try to partition enough that no partition is over 15gb,

Defrag is most important on the partition/drive that has the OS,software. There's thousands of little bits,10kb files and such,there's lots of change and stuff accumulates. A partition of music files,flacs,Waves,even mp3 is going to be files of several mb,many grouped as folders-and periodically chunks of 250-1000 megs are getting deleted. The arrangement of all that won't have an effect on your OS/software on another disc.

If you are not using O+O or another defragger able to do background defrag-its good to startup in SAFE MODE-which keeps any processes from interupting. Generally I just try to clear the old stuff off a storage drive and defrag maybe twice a year.