Could this be a phony site and they are looking for sensitive info from you (like your CC # to buy "genuine microsoft" downloads? I am thinking it is spyware/adware or whatever it is called especially if it keeps "popping up" like that.
I would run some anti-spyware programs and the like to try and get rid of it. I would not click on any links on that site. Maybe also go to the actual microsoft.com and email them to see if this is a spoof.