From the Adobe Forum
Then right click over both 'Flash9b.ocx' and 'FlashUtil9b.exe', and chose properties.
In properties choose the 'security' tab > Click on the 'everyone' account, and the button called 'edit' and then tick the box called 'allow full control' and also choose your own Windows local account name.
Once you have done this for both files run the FlashUtil9b.exe and it should install and update and tell you to restart. Do so and you should find that YouTube and other sites that use flash now work.