It may be inserted in your start up script. Use C-Cleaner Free, do a scan to clear your cookies, then do a scan to clear your registry of any errors. After running those two you can open up your Start Up and it will show you all of the scripts (probably not right phrase) that fire up when your computer does. You can pick out the offending line and delete it.
Only use the free in left column to get the free version. Read options carefully or it will also install a free version of "Avast" security. Make sure to uncheck that box before proceeding.
Malware bites is ok. I use Windows Defender Antivirus which does a good enough job at keeping gremlins away and mostly rely on AdGuard AdBlocker to keep these things from being inserted to begin with when online. That is found in the Google Apps Store, it's free also. I don't see ads of any kind on GON with it running.