You don't say how your existing desktop, in a separate room from the modem, is currently connected but presumably you have an ethernet cable running between the two.
The hardware that you will need to add a wireless laptop is a "Wireless Cable/DSL Router". This will connect by ethernet cable to the Cable Modem's ethernet port and your existing computer can connect to one of its LAN ports. A wireless enabled laptop can then connect to the router wirelessly.
The cost of a wireless router should be around £50. Be very careful if you are using Cable Broadband *not* to get a combined "Router & ADSL Modem". These are only suitable for use with ADSL delivered via the phone line and cannot connect to a Cable service.