What is your operating system and service pack and how much RAM do you have installed?
This issue, if not infection-related, is often caused by McAfee clashing with some other software. Are there any clues in your Event Viewer for instance? Do you have any other software that is protection-related?
Searching the forums brings up many instances of this, the most recent included the discovery that Nero 7 clashed, in particular InCD, and once it was disabled in Startup things improved.
As this is a holiday, help is going to be a bit sparse in the forums I feel so it may behoove you to try Technical Support Chat, free, via the McAfee Support link at the top of this page.