Have you been using any other security products at the same time, or employing System Restore or even registry cleaners? All of those can cause problems with the software.
Also what is your operating system and service pack?
Try uninstalling anything 'McAfee' via Control Panel/Programs/Uninstall a Program (Add or Remove Programs in XP)
Then run the MCPR cleanup tool listed under Useful Links at the top of this page. Reboot.
Reinstall from your online account.