On the non-working machine...
It should have a wireless card and a device driver should be installed.
Use the control panel to navigate to NETWORK Connections - the channel between the machine and the router is not working.
You should eventually see a window showing Local Area and Wireless icons. Right click on Wireless and select View Available Wireless Networks - when I do it I can see four - three of them in nearby houses.
Select the one you want (your own router) and all should be OK. You need to tell the wireless card and its driver which specific network you wish to connect to.
Good luck...
Peccavi
April 2009