I think you can write to the NTFS partition if the Windows computer "serves" it to the Mac, but not if the Mac mounts it. Macs cannot write to NTFS partitions directly. Basically, in the former setup, the Mac asks the Windows computer to write to the partition, and it does it on behalf of the Mac. That way, the windows computer makes sure the NTFS journal is correct, etc.
What you want to do is right click the folder(s) you want to share (or the root drive) and edit the sharing tab. At least, that's the way it was up till Win2k. I think it's the same in XP.