No, it's not necessary to input a cd-key. Wether you put one in or not, you're still gonna have to activate your windows within 30 or 60 days (not sure). Disabling updates is never a good thing to do with windows as a lot of them are security updates. Meaning the lesser the updates, the more vulnerable the windows. So when you have to install windows again, make sure to use one of the versions we've listed. A pre-activated one.
Then again it could be your cousin already gave you a pre-activated one rendering all of the above useless. You can check this by clicking the right mouse button on Computer and select Properties. It should be displayed at the bottom.