but things are quiet, and I might as well respond to your questions without moving the thread.
1. Ideally you would use a wireless modem/router. This would connect to the BB phone socket, and to the 'main' PC via its network card.
2. The second PC will need a wireless network adapter - either an internal, PCI card type (best), or an external, USB type.
3. Ditch your existing BB modem - you'll be happier with a modem/router combination, and they're not wildly expensive (around £50 - £70)
4. If you use a wireless router you can access the broadband connection on the second machine, without the need for the 'main' one to be running.