This error might happen if there is an extra copy of Steam.dll in one of the game directories or in the system path.

There should only be one Steam.dll in your Steam install directory, typically:

C:\Program Files\Steam\steam.dll
or
C:\Program Files\Valve\Steam\steam.dll

The fastest way to find errant steam.dll files is to use your O/S search function from your Start button:

Start > Search > For Files or Folders > all files and folders

then enter steam.dll in the all or part of the file name: box and click on the search button.

You should only have one Steam installation on your system so you should only have one copy of steam.dll. If you find any others then delete them.