Go to Control Panel > Add/Remove programs and delete all references to Adobe Flash. (Normally 2 entries listed as Flash Player Active X and Flash Player Plugin).

When you have tried to download flash from here you say it will not install.

http://get.adobe.com/flashplayer/

What error exactly are you getting that tells you it isn't installed?

Note: make sure you are using IE when installing it from that link above and not running Firefox at the same time.