You're looking for robocopy. I run:
robocopy E:\MP3 \\impee3\d\mp3 /PURGE
and it syncs up the two directories based on the source directory. The /PURGE option will delete files on the destination drive that are not found in your source directory. AFAIK, it only works on NT/W2k/XP, but you can give it a shot if you're on 95/98.
(You might have to rename the attachment to robocopy.rar to uncompress it. I renamed it to zip so I could upload it.)