Its a completely valid thing for Microsoft to do. If you do not buy the software then you cannot use it. It's not a free OS. If you want a free OS go download some flavor of linux. Otherwise if you want windows you buy it. Why wouldn't they try to keep out people who have pirated their software?
They could have made it auto forward all of ones information to the police, since technically they stole the software. I think annoying you with some pop-ups is a much more friendly way.
The downvotes tell me we have some pirated versions out there ;)