Where are you seeing this message, just visiting random web pages etc.

I would say visit the Microsoft update site, and let that do its validation test, that would be a true test to see if your version has been corrupted somehow, OR is it just "pop up" type thing like all the "your computer may be infected by a virus" etc, as betting it probably more than likely something like that, you got something on your PC, rather than it changing to an unregistered copy

Steve