I answer this one roughly weekly!
Flash Player has stopped working on your computer. (It just has a nasty habit of doing that; don't ask me why!). So you're told to download it. But your computer sees that the program is already there and doesn't properly install it.
Here's the solution:
Go to Add or Remove Programs (or Programs and Features, depending upon your version of Windows) from the Control Panel.
Find and remove Adobe Flash Player Plug-in
RESTART YOUR COMPUTER.
Reinstall Flash Player from here:
http://get.adobe.com/flashplayer/
Chris