Well, the linux box was sending packets too. After the mac spoof, both machines behaved as before. I'm sure the linux mac was spoofed, cause when i looked at the arp table on the router, it listed the 2 IP adresses (one of the linux box, the other one of the w2k box) both with the same mac adress. I just don't understand that if a switch his "routing table" is based on the mac adress it is still able to work properly when it has 2 mac adresses that are the same. Only explanation could be that the switch somehow got hold of the real mac adress. So my spoofing was not effective.

Anyway, much thanks on the replies, and thanks for the great tutorial !