Had the same issue with my 820 after a hard reset due to Other Storage problem.
The phone would not be recognized by either of my desktops (Windows 7x32 (laptop), Windows 7x64, windows 8x64). Drivers fails to be installed but the phone charges when connected to the pc.
The solution: did a Soft Reset by holding Power+Volume Down untill the phone restarts (no data is lost)
After the soft reset, I connected the phone to the pc and it launched the Windows Phone App and asked if I wanted to sync the phone with the computer.
Being working fine after that on all my computers.