In the upstairs PC, when you scan for "Available Wireless Networks", does it find the SSID (network name) of your Belkin router? If so, what happens if you highlight the SSID and select the option to "Connect"?
I have a Belkin USB 802.11g adapter which I use with several PCs. In every case I have it installed with *drivers only*, i.e. *WITHOUT* the Belkin connectivity software. In XP, I then use Wireless Zero Configuration to manage the wireless networking. You may be able to do this if the drivers are in a separate folder on the Belkin CD click here