HOW TO GET A FREE ANTI VIRUS

Defender isn't the best but in the years I've had it, not one virus. But I don't torrent apps etc.

Apart from Windows Defender (included with Windows) the usuals are the free versions of Avast, AVG or Avira. Where? From their makers site.

I keep using Avast Free Antivirus.

Yeah, like they said. Windows defender is pretty sufficient.

The Windows Defender Security center provides a robust and secure environment for the computer system. It is inside Windows which includes the Windows Defender. The Windows Defender saves your system from malware, viruses, spyware, and untrusted apps.
I personally trust on the defender and haven't seen any issue till now. So It is quite enough for the security of your data.

As long as you are using a genuine version of windows, Defender will be sufficient.
The only thing you should keep in mind is updating it online regularly.

As from your question I can understand that you don't wish to update your antivirus but without updating, you won't have protection from new viruses.
The database needs to be updated in order to keep you protected.
Moreover, an antivirus is of no use if has not been updated recently on a computer that uses the internet.