It sounds like when you downloaded Reader the checkbox was on to install McAfee, which is why this is happening.
The McAfee website says that there are two ways to remove McAfee Security Scan Plus:
You can also uninstall through the McAfee Security Scan Plus folder:
- Under Start menu, go to all Programs
- Open McAfee Security Scan Plus folder
- Select “Uninstall”
If this keeps happening to you, I would uninstalled Reader and then download and reinstall it from here:
http://get.adobe.com/reader/enterprise/ - this does not use the download manager, nor contain any bundled software.