1. USB modem (internal or external, it doesn't matter) connected to the ADSL filter at one end and to a computer at the other end.
2. A network adapter (internal or USB external) on each machine.
3. Connect the network adapters with Cat5 network cable - RJ45 plug at each end.
4. Run the Networking wizard on the XP machine, and tell it to make a floppy for use on the other machine.
5. Run the floppy on the second machine, and follow the prompts.