I am surprised it was faster on a WXP machine with a 100Mbps card as 11.9MB/s is just a bit faster than a 100Mbps connection can handle.

Have you tried the alternative driver, although I don't think it will necessarily improve anything?

How were you using the WXP machine? Did you just share a folder?

I know very little about samba tuning and many references on the internet are obsolete. One thing you can do is try turning on smb2. It may help for small files. Add the line:to /etc/samba/smb.conf. I've added it under a section called "# Other handy directives" but I may have added that as well! You may be able to reload samba with a "service smb reload". If not then you'll need to restart it.

Another thing worth checking is for network errors:Are there many TX or RX errors?