1) Is that a dreaded USB adapter, or PCMCIA?
2) Are you certain that you need an external adapter? Is your laptop not wifi enabled?
3) Please tell us what firewall each PC has.
4) How have you set up the adapter? Driver-only, or run by software? Driver-only is often best if you have troublw with a USB adapter.
5) Have you run the Windows Network Setup Wizard on both PCs?