Setting Up Network With Old Machine

I have a cable internet connection via a Motorola SB5100 Modem. This is currently connected to an old desktop PC running Windows ME via a USB Cable as there is no ethernet connectivity thro the PC ( But there would be a facility to plug an ethernet cable into the modem itself). I have just obtained a Dell Laptop operating on Windows XP Pro and would like to set up a network to share internet access only between the two machines. The laptop is Wi-fi enabled. i'm not too bothered whether the Desktop PC is connected the the net wirelessly or with cable, but obviously I want the Laptop to link wirelessly - what is the best way to set this up - ideally without having to fit the desktop with any ethernet adapters or similar?

Thanks

 

Thanks very much for your advice