You can repair your antivirus for that you should have Admin rights
then go to control panel and select SEP11-you will see a change or remove button-click on change-click next-select repair-follow the next steps
after completing your antivirus will work
if not scan your hdd in secondry with another antivirus
your system if is infected then you should clean it first then try to install the antivirus
if still you face the same problem you can scan with Mcafee SDAT which cleans your system thorougly.