When you say "I have a NTL modem box" is this the set-top box? If so just unplug the Cat5 cable and plug in the router. You can leave the desktop downstairs if you wish or take it upstairs the wifi router will find it.
You can use a USB adaptor if you wish on both the desktop and laptop but a PCMCIA card on the laptop makes life easier. You will be surprised how easy this is to set up, the installation disks now come with a full A to Z guide and you cant really go wrong.