Is your system up to date? What operating system and service pack are you using and what version of Internet explorer is installed (whether or not you use it)?
Were there any warnings posted?
It's possible, especially if this was an upgrade from a previous installation, that it corrupted somehow.
Try uninstalling anything 'McAfee' via Control Panel > Programs > Uninstall a Program (Add or Remove in XP)
Then run the MCPR cleanup tool and reboot.
Reinstall from your online account.