Because there's a registry error on your computer. (It's a very common problem).
Go to 'Programs and Features' (or whatever your version of Windows calls it, via the Control Panel). Locate and uninstall 'Adobe Flash Player Plugin'. RESTART YOUR COMPUTER. Go here
http://get.adobe.com/flashplayer/
Reinstall Flash Player (taking care to deselect the option to install McAfee Security Scan Plus at the same time).