If you do not delete cookies and other quarantined files regularly the number of files in the quarantine folder can become very large and deleting the files from within the Security Center takes a long time. Your system will be unresponsive for most of that time.
A better way to delete the files is to find "Access Protection" in the Security Center settings and uncheck the box. Then open Explorer and navigate to (this is for XP, and may be different for Win7 or Vista) :-
C:\Documents and Settings\All Users\Application Data\McAfee\VirusScan\Quarantine
Right-click on the Quarantine folder and make sure the Read-only and Hidden boxes are unchecked;
in the folder select all files except one called "mcqt.txt" (if it's not there don't worry, but it's in my directory and I don't think it's a PUP so I decided to leave it. I don't know what it does.)
Delete all the selected files. This is much quicker than deleting them from Security Center.
Restore the properties of the Quarantine folder to what they were before.
In Security Center, check the Access Protection box.