Slow Opening on Server 5.5, Client 6.0

[Orsi]

I am deploying a system that comprises 95 files on the server. So far everything was just running fine on many customers I have. This week I am deploying on new machines (client) running xp sp2 and when I try to open very simple files it takes too long. The odd thing is that when I change the subnet mask from 255.255.255.0 to 255.255.0.0 I get normal opening speed, but this is causing slowdown on mapped drives and other things. Can anyone help me please.