1 - according click here your laptop has wireless lan built in. That's good!
2 - I don't know much about your modem - I don't think that it is a router also.
3 - Get something like the D-LINK DSL-G604T WIRELESS AIRPLUS G+ ADSL BROADBAND ROUTER
click here= modem / router.
4 - you will need an ethernet cable (included) to connect the modem to the desktop. Once up and running, you can set up the wireless connection.