Vista Activation Key suddenly invalid?

Alright, so I'm just going about my business as usual on the computer. And I get a message asking to validate windows. Well at first I ignore it. I mean I've been using this computer for 2 years now. If the activation had been invalid, I'd know.

But then my wallpaper disappears. And at this I get VERY worried.

Now, I've put back in the product key# (luckily it was on the side of my pc), and it's back to normal. But I'm worried about why this happened.

There seems to be several of these situations after the last updates were received.
MS has started a new system to detect unactivated copies of windows, which is included with the updates. If your OS was activated with a valid key, just forget about it.

A thread on a similar situation
http://www.sevenforums.com/windows-updates-activation/103522-my-copy-windows-7-may-not-genuine.html