Although there's already been a post about the error 'MountAppFilesystem() failed: SteamMountAppFilesystem(4000,0,0x7c4fd24) failed with error 1: No default filesystems for this App.', there are many other ones that you may encounter with Steam. I'm just here to say that the majority of the time, the problem is that a file named ClientRegistry.blob is corrupt. To fix this, head over to your Steam folder. By default, these are the folders for Steam: For 32-bit users, you will find it at: C:\Program Files\Steam\ For 64-bit users, you will find it at: C:\Program Files (x86)\Steam\ Once you've found your Steam folder, simply rename or delete the ClientRegistry.blob file and a new one will be generated. I'd say this fixes quite a lot of problems with Steam, and is always a good idea to try this out.