You may have a problem with compatibility between the BT kit and the Belkin. I have had problems with a Safecom mini-pci WiFi card connecting to a Toshiba Access Point whilst a Broadcom mini-pci WiFi card worked perfectly.
Or you may need to reposition the WiFi antennas to improve matters. Try moving the Belkin about as far as you can.
I have also downgraded my 802.11g 54Mbps to 802.11b 11Mbps to improve reception and it's certainly better. After all 11Mbps is plenty fast enough for a standard ADSL connection. My WiFi and Ethernet connections perform identically when connected to the Internet (BT Broadband).