I have a similar setup on my computer network also. Basically four computers connected to a hub, three are win2k's and the one that I'm working on right now is xp-pro. My "server" is one of the 2k boxes. I have Zone Alarm Pro running on it, an internal DSL modem, and an ICS program, called Sygate.
One thing I would check is "ipconfig/all" from the command line, and verify that the DNS servers on your computers match the DNS servers that your ISP issued; they generally never change. If you have to add DNS servers (at least one), you can add them by going to control panel, network connections, and choose your local area connection. Then right click on it, go to properties, choose internet protocol (tcp/ip), click properties, click the radio button to manually enter dns server addresses, and then enter the ip address of at least one dns server your isp gave you. If there are two, then enter them both.
I'm not sure if my computer would slowly connect to only certain web sites without this DNS settings, but it basically takes a long time, and I'm too impatient to see if it connects at all.
Hopefully that will help you out.
Also, make sure that you are sharing that internet connection, and if you have a firewall (recommended especially with DSL), make sure that it is allowing outgoing and incoming traffic for web access.
Sorry for the long winded message, I tend to be very well detailed on my posts, especially if it deals with tech support.