How did McAfee Security Scan Plus end up on your computer? Well, If you were to download Adobe Reader from adobe.com, it will be bundled along with the install.
McAfee Security Scan Plus is a free diagnostic tool that actively checks the anti-virus, firewall, and web security status of your PC. It complements any existing security software and does not impact the performance of your computer.
This is the reason why I always encourage users to read before they click and also to use the Custom options when installing all programs.