Not sure how to do this on a PC.  But on a Mac you can set a Network Preference to share the
Internet connection.  I think then you can connect a wi-fi router to the laptop's Ethernet port and
have the router send out the wi-fi signal.

-Jason

You should be able to invoke Internet Connection Sharing under Windows, and
make an ad-hoc connection between the two WiFi adapters.

http://www.microsoft.com/windowsxp/using/networking/expert/bowman_02april08.mspx

Making the Wireless Home Network Connection in Windows XP Without a Router
"We'll start with a single computer that already has a wired Ethernet
broadband connection to the Internet. Then we'll build the ad hoc wireless
network in three steps:"

Just substitute your cellular card where they refer to wired.