in fact it's happening to a client of mine at this very moment, so you're not alone.
Make sure the laptop's firewall is turned off when you connect to the router, and ensure that the desktop is online before you start up the laptop.
Otherwise I can't offer a magic solution - yet. My client's machine establishes the network connection perfectly while I'm there, and as soon as my back's turned it goes back to its bad habits. She swears that I'm doing something special to make it work whilst I'm in her office, but I'm not. I can even connect my own laptop to her network without any problems at all.