Several ways...... The best would be to install an ethernet card on the Desktop, setup the modem with it and use a Router to Network to the laptop via Cat5 cable.
You can hook the Modem to the laptop and use Direct Cable Connection (DCC) with a USB cable to enable networking and Internet Connection Sharing (ICS). This method has much slower data tranfer rates than the first option but is a little cheaper.
Here's a good place to learn more:
brad
