OK. On one side, we have the office network with an ADSL connection, two hub, a watchguard firewall and an NT 4 Server. We have 5 licenses for Remote Users to connect through a VPN. Remote users authenticates on NT Server. Anybody with the correct key, software and password can logon with a 56k modem, browse the network and even do Remote Desktop connection on our XP machines.
Anybody except me and a bunch of people here. Why you ask? DSL. As soon as we use a DSL line to connect, things get bad. We can successfully authenticate on the server and browse a little bit but not much. What's more, we don't see the network. We need to know the exact IP of the machine we want to get to. Remote desktop is an utopic since we only get a blank blue screen and a "connection broken timeout network screwed-up" type of message.
But then again, with a 56k modem, it work just fine.
Can someone explain me this??

Its your firewall doing its job.

Can you be more specific. Why 56k is ok but not DSL???