The free version of Malwarebytes should meet your needs:
https://www.malwarebytes.org/antimalware/
It's 'run on demand' (rather than 'run in the background') so you need to remember to run a scan with it every so often. (I suggest once per fortnight is fine unless you're already seeing problems that might be caused by malware).
NB: When installing it, take great care to remove the tick against the option to take a free trial of the Premium version (otherwise you'll eventually end up getting prompts to pay for it).