If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the McAfee program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the McAfee application and click on Uninstall.
After that, download and run the Consumer Products Removal (MCPR) tool, reboot your computer when you receive Cleanup Unsuccessful message and install again the McAfee antivirus.