What operating system and service pack (if applicable) is this and is it totally up to date including any parts you may not normally use, (IE for instance if you use another browser)?
It sounds like it may be corrupted (can happen if you've run registry cleaners or for many other reasons, read on) or there's an infection.
For the second try some software suggested in the last link in my signature below.
For the first try using the Virtual Technician to see if it finds and fixes anything. http://mvt.mcafee.com/
If that doesn't help try uninstalling McAfee via Control Panel > Programs
Then run the MCPR cleanup tool and reboot.
Then reinstall your software from your online account
Also check for other software that may have active protection as that can sometimes interfere with security software.
If nothing helps I suggest contacting Technical Support, it's free by phone or online chat and linked under Useful Links at the top of this page.