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).