Can't find steam.dll error

The error message Can't find steam.dll relative to executable path <steam path> is caused by moving or deleting files in the Steam directory, or the Steam directory itself. If you cannot fix it by replacing the files that you've moved or deleted, you can first try to Reinstall the Source SDK installation. Next, you can try deleting the ClientRegistry.blob file and restart Steam. If that does not work, Steam itself must be installed from scratch.

Additional information

The Steam directory is where the steam.exe and steam.dll files are located. There should be only one copy of these files. Additional copies (due to an aborted install or other reason) will result in errors.