Avast probably comes out best in independent tests of free antivirus programs more often than anything else. (It's what I use):
https://www.avast.com/en-gb/index
You should top it up with the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
Note that the free version is 'run on demand', rather than 'run in the background'. That means that it will do absolutely nothing unless you actively run it from time to time - perhaps once per fortnight.
Note also that, during the installation process, you should take care to deselect the pre-ticked option to take a free trial of the Premium version.