I had Avast running in Windows 10 but it's not necessary. Defender is now enough. One problem, though. I couldn't make Defender turn on.
Here's what to do:
Delete conflicting Registry Entry
Some malwares adds malicious entries in registry that blocks real antiviruses from running. To remove these entries, follow these steps:
Press Windows key + R. This will open Run. Alternatively, you can go to Start and search for 'Run'.
In Run dialog box, type 'regedit' and hit enter. This will open Registry Editor.
In Registry Editor, navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options.
In this key, if you find any entry for MSASCui.exe, MpCmdRun.exe or MsMpEng.exe then right click on it and click on Delete. If you do not find any of these entry then it is normal and you don't need to do anything.