Steam thinks the game is not installed

I've researched the web carefully and went on about 20 pages, either from Steam itself or from this website or among other forums. Since I'm a StackOverflow user, I decided to give here a try:

I have steam on my PC for a long time. Not sure if I re-installed Windows at some point or not, but today I wanted to go back to play a few games so I managed to get Steam up and running again (D:\Games\Steam, deleted the ClientRegistry.blob and ran the steam.exe as Admin.) Apparently, Steam is running fine, it even recognize that I have Dota 2, Free to Play (the documentary) and Rocksmith installed (though needs update). But the one game I want to play (Left 4 Dead) it says it's not installed.

- If I try to install it, it gets stuck HERE:

- I can't repair it because ...

I'm looking for ANY ANSWER AT ALL other than

delete the files of the game from [...]\common\steamapps\ and re-download everything

. Since Dota 2 and other games were easily recognized (automatically), I was hoping there were something I could do to avoid downloading billions of bytes again (it will take me more than 24hrs to re-download everything).

Solution 1:

You could try this:

1. Move the game folder to a different location.
2. Click on Delete Local Content for the game in Steam if available.
3. Initiate the installer.
4. Once it begins downloading, pause and close Steam.
5. Move the original game folder back into where Steam is downloading the new files.
6. Run Steam and if it doesn't automatically do so, verify file integrity.