If the router is broadcasting then (assuming Windows Vista):
Before we start, look at the underneath of the Wireless Router and write down the following:
Identificador (SSID) - This should be WLAN_xx (where xx is a two digit Alphanumeric E.g. 19, 3C, BB. etc)
Clave - This is usually a 13 digit Alphanumeric password that allows you to connect to this routers Wireless Network.
Now:
Click on the start button and then click on control panel (bottom right on start panel)
In the Control Panel window, click on Network and Internet (middle left)
Now in the Network and Internet window, click on Connect to a Network (to the right, just under Network and Sharing Center)
You should now see a list of available Wireless Networks and this should include the WLAN_xx identification of your Wireless Router. Click on it to highlight it, then click on the Connect button (bottom right of window).
Enter the 13 digit Alpahnumeric password and click Connect to Network.
This will work every time on the proviso that the Telefonica Router is 'as out of the box'.