Vista -> XP slow LAN transfer

Hello. I have a Vista Home Premium SP1 computer connected by a router to a XP
Proffesional SP3 machine.
When transfering files from XP to Vista, transfer is limited by hard disk
BUT when transfering files from Vista to XP, transfer speed is limited by my
internet connection speed (and is shared with internet - meaning transfering
files thru LAN slows down downloading files thru internet).

There is no firewall active on any of the computers. Discs are defragmented.
Both cables and router are allright (when I had another computer with XP
connected, speed was normal). Tracert reports that the route is straigth to
the other computer.

So, please, how do I make Vista know, that LAN and internet connections are
separate ?
Thank you.