OK, thanks for that. I reset the WRT54GL, gave it the address 192.168.1.253 (the LAN uses 192.168.1.x, mask 255.255.255.0, and has its own DHCP server), disabled DHCP, changed the SSID to "abc", left security disabled (to keep it simple for the moment), then connected it (using in turn both the router's Internet port, and one of its LAN ports) to the Ethernet switch. Then I put a new Windows XP laptop (with a built-in wireless adaptor) beside the router and tested for available wireless networks, but it reported that it could not find any.
Does anybody have any suggestions about where to go from here?
Thanks.