Is it OK to run Windows Defender with Norton or McAfee antivirus protection?
You should never run more than one antivirus program at the same time. The two programs could slow down your computer, and they might even identify each other as a virus, which could lead to file corruption or other conflicts and errors that make your antivirus protection less effective—or not effective at all.
We recommend that you use the antivirus protection that’s included in your version of Windows. Windows 8 includes antivirus and antispyware protection called Windows Defender. If you use Windows 7 or Windows Vista, you can download Microsoft Security Essentials at no cost.
For more information, see How to boost your malware defense and protect your PC.