Cannot install OSMC: Unable to start_cd.elf

I wish to install OSCM on my Raspberry B. Following the installation instructions, I have downloaded the img file "OSMC_TGT_rbp1_20150412, unpacked it, put a 8GB SD card in my card reader, but cannot start the START_CD.elf file. My System (WINDOWS 7 Home Premium, 64bit) does not “know” this file.
Now, there are a number of programs in the internet, which will enable WIN7 to read .elf files. However, most if not all are costly (more expensive than the PI) and try to install other apps, PC cleaning etc.
Is it really necessary to buy one of these progs (for just one transaction !) or is there another way.
I scanned through the forum and was surprised that nobody else hat a similar issue. Maybe I am on the wrong track.

Why are you unpacking the image file? Use win32diskimager or similar to write the image file to your sdcard, place sdcard in pi and boot.

This would all be done for you if you had just downloaded the windows installer and not the standalone image file.

I was totally mislead by the instruction I found in the Internet (ARCOR). It was for Raspbmc-which does not seem to exist anymore. However, everything worked fine with the Windows Installer - no need for downloding anything else. Thanks a lot.