Normally you would simply delete them from within SecurityCenter
Click Navigation then Quarantined and Trusted Items and click those sections to expand.
However, sometimes there are so many that it's quicker to do it Windows.
What operating system is this?
So it must be Vista or Windows 7 then.
First double-click the taskbar icon to open SecurityCenter
Click Navigation (top right)
Click General Settings & Alerts (left)
Click Acess 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.