All of a sudden, without touching anything (in fact, without booting Windows during last few days) I got this upon trying to launch Half Life 2:
Failed to load the launcher DLL:
The specified procedure could not be found.
Verifying files did not help me. The solution (for me)? I went to:
C:\Program Files\Steam\steamapps\ivucica\half-life 2\bin
and I moved all the .dll files away. Then I relaunched the game. Steam happily recreated all .dlls, and the game now works. I safely removed old copies of the DLLs.
Since I didn't think of making a screenshot, image from http://www.hl2world.com/bbs/-vp666240.html