Please let us know some of the basic information regarding your computer ?
What is the operating system ? (Windows Xp/Vista/Win 7)
Do you have any other security software besides McAfee ?
Did you recently perform any changes to your computer like a system restore : (restoring the computer to an earlier date)
Please click here : http://mvt.mcafee.com and run McAfee virtual technician and report back your session id no from it.