Unfortunately, Windows NT 4 and Windows 2000 do have a restriction for the implemented Defrag API -- they only allow NTFS partitions with a maximum of 4K cluster size to be defragged. Win XP does not have this restriction anymore.
So there are no tools available, that rely on the MS defrag API, to defrag your volume. There is "SpeedDisk", which does have its own defrag API implementation, and thus can defrag any cluster size. But beware! This tool does not comply with NTFS security and data safety -- it defrags the MFT online! You should do a full backup before applying it to your data!
This was first published in September 2002