If you need to uninstall an old antivirus program you should first try to do it using any uninstall link provided with the program. If there isn’t an uninstall link go to Control Panel-Add/Remove programs and uninstall it from there.
Sometimes that won’t uninstall it properly (antivirus programs have a bad history of not having good uninstallers) and you may need an additional tool to clean it properly. You can get one here for most products: http://www.computerhope.com/issues/ch001402.htm
Please do not run more than one real time scanning antivirus tool on your computer at the same time – it would slow it down and it can sometimes cause additional problems including failure to properly uninstall antivirus tools!