Server 2003 Not letting me access internet.

and 3 dell workstations running MS XP Pro. Ran the quick setup feature for the server, setup all users and all workstations with no problem. Everything connects fine. Problem is there is an existing Bellsouth DSL line that is run through a cayman router, and the rest of the connections is connected through a switcher that is also connected to the router and server.
Here is the problem, when the server is connected to the hub, workstations get on the server fine, but can't access the internet.
Unplug the server from the hub, and the workstations accesses the internet with no problems, obviously not the server though..
But hook the server backup to the hub, then most times everything is working at same time, not all users, and not all the time. Key is to boot workstations while server is disconnected, then boot up.. We've had to operate this way for the last 2 days til we get a fix. Any sussestions... I've read a few things about IPSec, thinking this may be the answer, but would like some imput from someone more knowledgable with Server 2003..