This is bizarre. It happened a month ago, and I thought I must have done something to cause it. But now it has happened again, and I know I did nothing.
I open Explorer to check what computers are on the LAN. I see that I'm the only one. Fine. Then I expand my computer name, to discover not only my shared folder, but my entire User hierarchy on the C drive. I right-click the User folder and check, and indeed it is Shared. I break the sharing and it disappears off the network.
Obviously, this is a massive privacy leak (theoretical in my case, but nonetheless...).
I have scanned using several programs, and the system is clean.
Windows 7, hp, 64b.