Suddenly, my copy of Windows is "not genuine." [View all]

I have an old, old laptop with Windows 7 Professional. I've been using it for years.

This morning, I get a message "you may be the victim of software counterfeiting," and there's a note on my screen that says "this copy of Windows is not genuine." It doesn't seem to be affecting anything, but WTAF?

The Registry number has long worn off the label on the bottom of the laptop. I did a system Restore, but they're still telling me Windows is not genuine. Any ideas on what I should do?

-- Mal