What to do if your antivirus software stops working
Most antivirus software will warn you if it is no longer working. For example, Windows Defender and Microsoft Security Essentials both provide warnings if they have been turned off or aren’t working. If you see a warning that your antivirus software has been turned off or disabled, there are several things to look for that might be causing it.
Watch out for fake warnings
If you see a warning that your antivirus software has been turned off, it might be a fake alert, also known as “rogue security software.” This type of warning is designed to fool you into downloading malicious software or paying for antivirus software.
If you use Windows, you do not need to pay for antivirus software. Windows Defender is included in Windows 8, and Microsoft Security Essentials is free to download for Windows 7 and Windows Vista.
These kinds of fake alerts could mean that your computer has a virus. Do a free scan with the Microsoft Safety Scanner to see if it detects rogue security software.
For examples of rogue security software, see our Real vs. Rogue Facebook test.
Do not install more than one antivirus program
Your antivirus software could turn off if you try to install another antivirus program. For example, Windows 8 comes with Windows Defender antivirus software already installed; if you try to install another program, you might see a warning.
You do not need to install more than one antivirus program. In fact, running more than one antivirus program at the same time can cause conflicts and errors that might reduce the effectiveness of your antivirus protection—or eliminate protection completely.
Windows 8 includes a more advanced version of Windows Defender that provides enhanced protection against viruses and spyware. If your computer is running Windows 8, you don’t need Microsoft Security Essentials. If you install a different antivirus program, Windows Defender will be disabled automatically.
Windows 7 and Windows Vista include spyware protection called Windows Defender. You also need antivirus protection, which is not included in any operating system before Windows 8. To get antivirus protection from Microsoft, download Microsoft Security Essentials, a more powerful program that helps fight both spyware and viruses. Make sure to uninstall any other antivirus software on your computer first (whether you installed it or it came preinstalled). If you download Microsoft Security Essentials, Windows Defender will be disabled automatically.
You might have a virus
Some viruses can disable your antivirus software or disable updates to your antivirus software. Viruses can also prevent you from going online to update or reinstall your antivirus software.
If you think your computer is infected with a virus and you can’t connect to the Internet, use another computer to download Windows Defender Offline. After you have downloaded it, you can create a CD, DVD, or USB flash drive and use the media to run Windows Defender Offline on your infected computer.
Windows Defender Offline helps protect against advanced malware that can't always be detected by antivirus software. It works with Windows 8, Windows 7, and Windows Vista.
Learn how to use Windows Defender Offline
If you’ve tried the suggestions above and your antivirus software is still turning off, you can get support at the Virus and Security Solution Center or at Support for Microsoft Security Essentials.