Check your browser Tools/Manage Add-ons to see if it has been accidentally disabled.
Go to Control Panel/Add or Remove Programs (XP), Programs/Uninstall a Program (Vista) and, if it is listed remove it.
If you see only "McAfee Security Center" listed, click the uninstall button for that and, when the subsequent menu appears asking what components to remove, if it is listed remove it.
Reboot then re-download either from you account (if it's part of your suite) using the Customize option in the installer. Only install it.
Or go to
http://www.siteadvisor.com and download it from there.