It's a known issue, when there are too many the deletion jams up. You can physically remove them from within the Windows environment.
First double-click the taskbar icon to open SecurityCenter
Click Navigation (top right)
Click General Settings & Alerts (left)
Click Access Protection to expand that section
Uncheck Access Protection and click Apply
Leave SecurityCenter open on your desktop because you should re-enable Access Protection after the following steps.
Go to C:\ProgramData\McAfee\VirusScan\Quarantine and click Edit/Select All
Click Shift and Delete simultaneously and the folder should empty.
Take care to delete only the contents of that folder, not the folder itself.
Re-enable Access Protection as mentioned above and click Apply and then exit SecurityCenter.
Those instructions are for Vista/Windows 7. For XP the folder is found at: C:Documents & Settings/All Users/Application Data/McAfee/Virusscan
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I forgot to add you can reduce their number in future considerably if you are prepared to accept Tracking Cookies. Those enable websites to remember your sign in details etc.
With SecurityCenter open click Virus and Spyware Protection to expand that section.
Click Real-Time Scanning
Click Settings and scroll down to Tracking Cookies
Uncheck that and click Apply.
Glad to help.