Networking from NTFS-drive going really slow

I don't have any good suggestions, but since you're connecting to the NTFS volume via the network, the format doesn't have anything to do with it--drive formats really only make a difference to the host OS, not one accessing the volume over a network.

Essentially the MacOS is seeing it as an SMB volume (Windows networking) regardless of its local format, and SMB shouldn't cause any problems.

The source of your slow transfer is either network weirdness or some sort of glitch in one or the other of the computers involved that's causing slow transfers. Most likely would be you have a hub that's only operating at 10Mbit rather than 100--that would lead to a 20GB transfer taking on the order of 5-10 hours. Have you checked to make sure both computers are connected at 100Mbit? (System Profiler or Network Utility on the Mac, I think the Networks window on XP.)