I have downloaded Adobe Reader X (version 10) on several PC's now, and the installer always installs McAfee Security Scan Plus with the reader. I've seen no way to opt out of the McAfee "trailer."
Further, as far as I can tell, McAfee Security Scan Plus is "advertising" software. Run it and you will eventually be prompted to download a not-free version of McAfee. It may have other functions, but I doubt that it has any value in a computer already employing appropriate firewalls and other virus protection.
Suddenly I've lost confidence in both McAfee and Adobe, especially Adobe. Now I wonder what else Adobe put on my computer that they DIDN'T tell me about.
I've uninstalled McAfee Security Scan Plus, but I'm still a little concerned.