XP network performance problem 2003 LAN

This is my second post as my first was probably a bit wordy. We have a small
situation where a customer of mine is trying to use PC-Anywhere and FTP from
home and from a remote office. When the connect to the office via CISCO L2TP
VPN or even via STATIC PUBLIC ip the users from home or remote office can
transfer files faster from a Macintosh or Windows 2003 server than they can
from ANY of the connected Windows XP computers on that LAN. So my question is
why is there a limitation on the XP-Pro-SP2 boxes in transfering files to
these remote users? All XP boxes in this LAN are SP2 running different
AntiVirus, Firewall programs. We even tried a virgin box for grins and the
same thing happens. When a remote user pulls files from these PC's either by
FTP or SMB the transfer is 1/2 that of the Mac OS or 2003 Server. And the
same goes for pushing file from the LAN to a remote user. Server and Mac fly
but 8 XP boxes get about the same performance...... WHY IS THIS HAPPENING????
I have not been able to find a single article on this. Please help I have
spent 3 weeks rebuilding routers, switch configs, and firewalls to test this
only to find it is an XP problem.

Thanks,


--
Devin M.
Network Engineer
Detroit, MI.