Microsoft Windows 8 has built-in antivirus software called Windows Defender so we don’t need to install Microsoft Security Essential as in Microsoft Windows 7 or previous Windows version. If you need to install other third party antivirus software such as Norton Antivirus or McAfee, etc… you need to disable Windows Defender first. You cannot more than one antivirus software run at the same time. It will slowdown your machine and make it easy to crash. I am much happy to use the Windows Defender on Microsoft Windows 8. It saves me at least fifty bucks per year on antivirus software.