This particular router has an install CD that comes with it. The router has to be installed to one computer and then any other computer that tries to access the router is given a pin that has to be input onto the computer that has the router installed to it. I don't have the install CD for this anymore so I can't add other computers to that router.
The pin number you are referring to is presumably the WPA or WEP wireless security key. This can be viewed, disabled or changed by accessing the router's user setup screens -- without needing the CD from my experience with something like 5 brands of router (though I haven't used Belkin I'm confident that they conform to the norm).
Connect the computer to the router with an ethernet cable.
See the literature which came with your router for how to access the user setup screens in the router or download the full manual from Belkin.
Once in go to wireless security and review or renew the wireless security type and password. WPA PSK TKIP is the most common security type in use.
Go to wireless mode and ensure that it's set to a protocol matching your computer's wireless adapter (mixed mode is a safe bet).
Disconnect ethernet cable.
Go to the computer's wireless networking setup screen and see if you can detect the router's SSID (wireless call sign). Select that network.
Apply the wireless security type and password to the computer's wireless networking setup screen.