Many cable internet providers now use "MAC address authentification". The MAC address is the Machine address of the computer (nothing to do with Macs in particular). Every ethernet card in every device, Mac, PC, printer, what have you, has a unique 12 character hex MAC address, something like "00:0A:12:34:5B:6C", for instance.
Many cable companies are set up so that the cable modem or their servers "memorize" the MAC address of the first device that connects to them and won't let any other device connect thereafter. That sounds like it might be your problem.
Depending on how the cable company has set things up, you might be able to get the modem to "forget" the MAC address by unplugging the modem for a few minutes before switching computers. This does not always work. As I said, it depends on the cable company's set up.
The best solution is to spend $20 - $40 for a Cable/DSL router with a "MAC address cloning feature". The most popular version is probably the 4 port Linksys BEFSR-41.
If you get a Linksys router, you can plug the cable modem into the router and then plug both the Mac and the PC into two of the other four ports of the router and surf the net simultaneously with both computers.
To get around the MAC address authentification problem, the router has a software setting in which you can type the MAC address of your PB and the router will then "spoof" that address so the modem thinks it is the PB that is directly connected.
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