Werid Net Share Problem

Hey, this problem is driving me crazy.

I have three home computers all running WinXP home, and an ADSL modem and Hub. I am sharing the internet with all the computers, all having to connect individually. Recently a strange problem has occured. To share the internet one of the computers must be connect to the hubs Uplink port. This wasn't the case before.. what is up? I checked all the computers for an internet gateway and there is none. But when I rearrange the network cables in the hub to have the adsl modem connected to the uplink port.. the net stops working. As well the computer that has to be connected to the uplink port, must also be connected to the net for the connection to be shared. What can I do to revert this back to a normal connection share? Thanks in advance!