|I've wasted a lot of time on Windows 7 file sharing and I've come up to the conclusion that it is acting weird, perhaps buggy. I could write an essay on the topic but I'll try to be short in this reply.|
A padlock on the folder icon means that the folder is not shared with anyone. It appears when you go to folder properties, Sharing tab, and then click on "Sharing..." button and then remove a user from there (which you probably added earlier). Then even if you click the button below named "Advanced Sharing..." and disable sharing completely, the padlock icon will stay. That is because when you removed the user from the "Share..." window, a group called "Authenticated Users" was removed from the Security tab. If you re-add it manually and set it to have all permissions except "Full control" and "Special permissions" then the padlock will disappear and things will be back to normal.
Windows 7 sharing also does not copy users added under "Sharing..." button to the "Advanced Sharing..." button and vice versa, as it does in Vista. If you remove a user from "Sharing..." button (and with that lose the "Authenticated Users" under Security tab) then the "Advanced Sharing..." button and everything related to that will not work at all.
At the moment to me this looks like a nasty bug in Windows 7.