I'm betting you using the lin98sk-driver (which is typical for built in
networkadapters on athlon64 mb:s). I think I pulled all my hair off
trying to solve the same problem a few month ago. It's just so typical
when you change all your hardware, upgrade the software and then run
into problems.
Here's the problem in a nutshell:
One samba-server (stand alone)
A couple of Windows XP (sp2) clients
All connected through a gigabit network
Transfers between two of the windows computers works just fine and the
speed is about 20-30MB/s.
Transfers from the server to any windows computer also work