1. Right click the game in the library, hit properties and select verify game cache. It should re-reg the DDL the file that is missing. (Internet needed)
2. Download Ccleaner and fix the registry errors (always save a copy when asked to), clean you PC in general and reboot your PC.
3. Type CMD and press Enter... Type "regsvr32 steam_api.dll" and press Enter.
Hope one works for you.