DeltaCopy

DeltaCopy is rsync for Windows. It’s open source and is available for NT4, 2000, XP and 2003. The idea of rsync is that if you’re trying to backup files on a regular basis, then a normal backup program would copy everything during every time interval. This is a waste of bandwidth. But if you only copy what has changed, then it’s more efficient use of the bandwidth. DeltaCopy accomplishes this by basically wrapping the rsync program with a Windows wrapper program.
Lifehacker < DeltaCopy