Today, I decided to reinstall Windows 7 on my HP Laptop. I never activated it and I lost my original DVD. So I had no choice than install it from an original Windows 7 ISO downloaded on the web.
And that's why I discovered that pre-installed Windows 7 keys cannot be used to activate Windows. So, this is the solution. Open a terminal as administrator and execute the two following commands :
slmgr.vbs -ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX (where XXXX... is your OEM key)