ok, did it come with a setup cd? It should just be a simple matter of inserting the adaptor into a spare usb port, windows will detect new hardware and ask to search for drivers ( software which makes hardware work) Insert the setup CD and load the drivers off it. Then the windows wireless software should kick in, you will get a little icon in your system tray that looks like a monitor with lines coming off it, right click that icon then choose VIEW AVAILABLE WIRELESS NETWORKS then click on your network to connect.
Or alternatively go here >
BT.com | BT Voyager ADSL , CLick the big button that says CLICK HERE then follow the instructions.