What's the setback of using a non-genuine copy of Windows 7?

I have a custom built PC that was bought from a friend and I recently had to rip and reinstall Win 7. I didn’t have a restore disc, so I had to use a copy I got from a work buddy. It’s been a month now, and Windows wanted me to activate my copy, which I could not do because I have no product key. Now it’s saying my copy of Windows is not genuine. I can still use it just fine, I can get on the internet and seem to have all the functions I need, so what’s the consequence of using Windows 7 without activating it?