The best way to know for certain that you have a genuine copy of Windows (and no-one is saying you haven't

) is to go to
Microsoft Windows Update (in IE) and see if you can get to the updates.
And the second question im not sure of but do you mean share a software licience? Im sure the copy i have was bought form PC World.
No, it's ok, I think we're dealing with a "Home" PC here aren't we?