elena, I have found McAfee to be very trouble prone and no longer use it.  The usual reason for the problem you are having is a conflict with some other program.  You can post on the McAfee forum or run the McAfee "virtual technician"  that may correct the problem automatically for you.  Quick Clean may work in safe mode which can be reached by repeatedly pressing the F8 key when you first start your computer.  Select safe mode from the menu by scrolling to it with the keyboard arrows and hit enter.  You can choose to be administrator and then run the McAfee program.  You may have to start McAfee  Quick Cean manually if it does not auto boot.