There's usually a cause for things that happen, which is why the first thing most computer support people (including me) ask is "what did you do just before it went wrong". Typically, the answer is "Nothing! Well, I did just install this program, but that wouldn't affect anything, would it?"Here
is a complete technical discussion of what has happened.
A common trigger, it seems, has been some version of Avast! antivirus (which is a shame, because it's the free AV that I used to recommend - I now recommend MS Security Essentials if people want a free AV solution; but if you're prepared to pay, ESET NOD32 is simply the best). Read about it here
. This is fixed in the current version of Avast! More recently, other AV packages have had the same problem with Vista - read here
. This also suggests that not having Vista patched up to date is a contributory factor.
Have you tried this reactivation procedure?
1: From the Start menu, select cmd window. If cmd application is not
visible type cmd in the "Start Search"
2: Right Click on the cmd window
3: Scroll down and select "Run as Administrator"
4: run the command "slmgr -ato"
5: Wait for sometime to see a dialog box
6: Most likely you will see a success page.
7: Visit http://www.microsoft.com/genuine website and validate again.
8: Restart your computer.