You'll need a wireless broadband router to achieve this.
Follow the link below for an example.
http://www.pcworld.co.uk/martprd/store/pcw_pag e.jsp?BV_SessionID=@@@@0546538716.1171878849@@ @@&BV_EngineID=ccchaddkeiefklmcflgceggdhhmdgmk .0&page=Product&fm=null&sm=null&tm=null&sku=88 2740&category_oid=
Basically the way it works is you have 2 cables running from the router: one to the NTL modem currently installed and the other to your PC. As the laptop is wireless you do not need a third cable going from the router to the laptop (but that is essentially what you are achieving)
It is fairly easy to set up but you must make sure of the following before using a router:
1. Your PC must have a network card installed (most modern PC's have)
2. You must make sure you follow the instructions on setting up wireless security - as I'm sure you won't want someone else connecting into it and stealing your internet!
Any more questions please feel free to ask!