Adobe Flash Player won't install properly if it finds that you've already got it on your system (even though your current installation isn't working). So the first thing you need to do is to go to Start > Control Panel > Programs and Features to look for any reference to 'Adobe Flash Player'. (There's often only one entry but I've seen two there sometimes). Uninstall those program entries.
Then RESTART YOUR COMPUTER. (That's in capitals because it's important!)
Next, go here
https://get.adobe.com/flashplayer/
Deselect the 'optional offers' and click to install, following any instructions given.
(Go to your list of installed programs again if you want to check that it's there again).
You should then find that everything is OK.