So you are right clicking and scanning the quarantined folder ...are you?
Maybe the folder is over full so open Mcafee and go to navigation and quarantined and trusted items and delete them. If that fails do a manual deletion
You can clear all the PUP and tracking cookies manually by going to “c:\documents and settings\all users\application data\mcafee\virusscan\quarantine “ in Windows XP and delete all the files inside the Quarantine folder. Make sure to disable Access Protection (McAfee >> Navigation >> General settings and Alerts>> Access Protection ) before deleting the files and revert back to active after deleting the files.
In win 7
Also there is no point in trying to scan that section as VirusScan has already scanned it and encrypted those files before placing them there for your protection. Scanning them again wouldn't change anything.
If something in there shouldn't be there than you would have to temporarily disable VirusScan, restore the item and try to get McAfee Labs to approve it.
See this article: https://community.mcafee.com/thread/2016