Fix: Check your proxy settings. Go to AV settings > Update > Proxy. Set to "Use system proxy" or "No proxy" if you are on a home network. Flush DNS: ipconfig /flushdns in command prompt.
If ransomware encrypts your system, your AV might be deleted first. Set a weekly offline backup (external drive disconnected after backup) alongside your AV. setting av app
Windows Defender automatically turns off when it detects a third-party AV. However, if you are setting AV app on an older machine, manually check that no remnants of old antivirus (like Avast or AVG) remain. Two AVs running simultaneously will cause a "Blue Screen of Death" (BSOD) due to file contention. Fix: Check your proxy settings