| The CPU running high, even at 100% is normal when a program is running and using the resources. As long as the CPU usage goes down when it is done, you are fine.|
McAfee (and Norton) are known for their poor usage of system resources. Although it is very important to keep out infections, their designers do not apparently believe that programs you launch and run should take precedence on resources over their program. This often means poor performance of your system overall when a scheduled scan or update begins. You may want to consider uninstalling McAfee and install Avast or MS Security Essentials (both available free - Avast remains free if you register it) or Webroot's Antivirus with Spysweeper (you need to pay for this one) since these use system resources more intelligently and actually work better. ((Note: You will need to download a special uninstaller from their site to completely remove McAfee from your system))
If you still have higher than normal CPU usage when your antivirus is not running or updating, let us know and we can suggest some things to shut down background running programs that you really do not need.
You have to be a little bit crazy to keep you from going insane.