Is the desktop running XP or Vista? You can set up Internet Connection Sharing (ICS) on the ADSL collection, which should be as simple as going to that connection, saying "Share" and following the instructions. Then, once you make the basic connection between notebook and desktop (wired or wireless, as described in other replies), the Desktop acts like a router, including handing out
IP addresses.
A friend of mine did exactly this, a few years ago - worked fine, the computers all saw each other and the Internet.