If your list of wireless networks looks like this:

I’ll show you how to fix it.

1. Use your favorite search engine to find the Sysinternals utilities for download from Microsoft. You need the movefile utility. It may come with the pendmoves utility also.
2. Unzip the download and open a command prompt
3. Type the following into the terminal (replace with your username, of course!):

movefile.exe "C:\users\<username>\Local Settings\Application Data\Microsoft\Windows\usrclass.dat" usrclass.old

4. Restart your computer. Your Manage Wireless Network list should now include your wireless networks.

Why does this work and what caused the problem in the first place? I have no idea.

I found this solution on this Microsoft answers website which in turn links to this Microsoft KB article.