Stealth mode is stuck on

Is there a way to reset the ApplicationFirewall? I have a Mac that exhibits the behaviour of being permanently stealthed. It doesn't matter if the firewall is on or off or whether the stealth mode is enabled or not, my Mac does not respond to pings or requests for file/screen sharing. Switching on file or screen sharing causes the Mac to appear in the Shared section of another Mac's finder but clicking on it does not permit a connection. The permissions are correct.

Before the problem occurred, when I connected my Mac to a predominantly Windows network, all of the Windows PCs appeared in my Mac's Shared section in a Finder window. Now I only see other Macs. I can connect to Windows shares using the Connect to Server command provided I know the server's name.

My firewall had this problem before and I solved it by reinstalling OS X. I don't want to go to this length - I just want to bring my Mac firewall back to normal behaviour. It appears something is stuck and therefore I want to reset the firewall back to the factory settings. I cannot find any way to do this.