Today I finally worked up the nerve to attempt to resecure the network. I had previously (a month ago, when I first set up the secure wireless network) found that using the router's web access page to change the name of the network had locked us out of the network, so I was afraid to try to use it again. Instead, I tried reusing the installation CD that came with the router. This software said all over it that it should only be used to set up the router for the very first time, and that subsequent modifications should be done through the router's web access page instead. I ignored these warnings . . . or tried to. I soon found that the software froze up and refused to cooperate.
Reluctantly, I returned to the router's web access page. I followed the instructions for changing the name of the network. Just like before, I was immediately locked out of the Internet.
After some fiddling (including a completely unnecessary repeat of pressing the button to reset the router to its default settings), I tried accessing the router's web access page from Susan's Mac instead of from my PC. When I changed the name of the network from here, I again lost the connection to the network - but from the Mac, it was perfectly easy to bring up a list of nearby wireless networks, select the new name of the network, and reconnect. I then resecured the wireless network with no difficulty at all - and because I reset it to the same settings that I had already set up my computer for when I first created the secure wireless network, I was able to connect from my computer with no problem at all.
The irritating thing is that my PC does not list the nearby wireless networks. This is what threw me off and prevented me from figuring out how to access the network under its new name until I had finished reconfiguring it on the Mac to exactly the same settings it had before. In the past, when I lived in my apartment, I remember that my computer did list nearby wireless networks. But now it doesn't. It looks like this has to do with the fact that the router's software is managing the connection, so Windows XP's default connection management screen is apparently not allowed to even touch it. But the router's management software doesn't provide a list of available networks! I would really prefer to have a list again.