If you find that the Yes button in User Account Control prompt is grayed out, even when you are signed is as an administrator, then here are a few things you can try.
1] Try to restore your computer to a prior good point.
2] Try a Refresh or a Reset of Windows 8 or a Repair Install of Windows 7.
3] Create a system restore point first. Then we will use the Net User command for administrators.
Boot in safe Mode and open an elevated CMD. Type the following to enable administrator account:
net user administrator /active:yes
Next reset the administrator password by passing the following command.
net user administrator password
Reboot your computer and log in again with your administrator account.
Now open an elevated command prompt and add your admin user account name to the Administrators group, by using the following command:
net localgroup Administrator <USERID> /add
Reboot and log in. See if it has helped.
If you don't like the results, restore you computer back to the just created point.
3] If your User Account is corrupted, you may have to go for a reinstall of your Windows.
Hope something helps you.