You probably failed to deactivate the license for the previous installation which can happen if you don't uninstall the software in the normal way, for whatever reason.
First sign in to your online account here: http://home.mcafee.com/Default.aspx?culture=EN-GB (top right) and go to your subscription.,
or here: https://home.mcafee.com/Secure/Protected/Login.aspx alter the Country at the top right if necessary.
Click on subscriptions and deactivate the license in use for the old installation.
The page looks a little like this: (Click to enlarge...this is a pic of my own account so product names may vary)
When you click Subscription it should open up to look something like this (I've named my entries so I know what they apply to but usually by default they bear the name you've given the computer at Windows setup.
Once that's done you should be able to activate from the user interface on your computer.