I renamed the wireless network (ESSID) of my router from one of my Windows XP PC's, and then found that my Windows 7 PC could no longer see any networks at all (there are several from neighbours in addition to mine).
Nothing seemed to help - Rebooting, uninstalling drivers
for the wireless card, etc. Eventually I got it to work by giving the network card a fixed IP address, and the it could see all the networks again. I could then change it back to DHCP and it was still okay.