Big problem Steam won't start and can't reinstall it

Hello, when I click on Steam (Windows 8.1) nothing happens, in the task manager I see it start but nothing occurs, and after about 10 seconds Steam.exe shuts down.
So I wanted to reinstall Steam by deleting all files except Steamapp and Steam.exe (a method that allows you to keep your game saves) but in the Bin folder, I’m told that the files cannot be deleted because they’re running.

When I go to the task manager and to the details tab I find “Steam.exe” and “Steamwebhelper.exe” (which doesn’t appear in the processes tab by the way) but I can’t close the process or end the process tree, it tells me “The operation could not be completed. Access denied.”
So I conclude that the problem comes from these 2 processes that won’t close!

So I don’t know what to do, I’m stuck with an unusable Steam that doesn’t launch any games and I don’t want to uninstall Steam and lose my saves.

Yesterday everything worked fine but today this cursed application is preventing me from playing my games!

Does anyone have a solution because I really don’t know what to do :/.

Configuration: Windows / Chrome 43.0.2357.130