Antivirus Compatibility

• All real-time antivirus programs, including Avast, install software deep into your computer's operating system. This level of integration allows the antivirus program to catch even well-hidden viruses, but it can also cause conflicts with other integrated software. Because of these conflicts, you should only install one antivirus program on your computer at a time. Running two AV programs at once may cause system instability or slow performance, or cause the programs to not run properly.
  
  

Uninstalling Antivirus Programs

• Because of the deep level installation of antivirus software, uninstalling AV programs through Windows' "Programs and Features" control panel does not always work properly. Most antivirus companies offer a special uninstall utility on their websites that will completely remove unwanted AV software. Before installing Avast, use one of these utilities to clear off any old antivirus programs. If you choose to go with another program instead of Avast, download Avast's uninstaller, "aswClear" and run it from Windows' safe mode.
  
  

Antispyware Programs

• While Avast will conflict with other antivirus software, this does not extend to other types of security software, like antispyware and antimalware scanners. Popular programs such as Malwarebytes, Ad-Aware and Spybot S&D run scans for types of infections that traditional antivirus software may miss. These programs will not conflict with Avast or each other, allowing you to use one or more of them to complement your antivirus software.
  
  

Internet Antivirus

• One potential reason to want multiple brands of antivirus software is to double-check the safety of a suspicious file. While installing multiple antiviruses will not work, many websites offer online virus scans that are compatible. To avoid scams, stick to websites of reputable antivirus products. You can also use the VirusTotal website, which scans a file using the antivirus definitions from dozens of different products at once. Note that Web scanners only alert you to a virus' presence, rather than repair it, so they should not replace an installed antivirus program.