Malwarebytes Anti-Malware continues working after the free trial has ended but only on a 'run on demand' basis, rather than 'running in the background'. Just remember to run it every so often (perhaps once per fortnight, or whenever you suspect your computer might have picked up malware) and you'll be fine.
If, for some odd reason, Malwarebytes Anti-Malware has ceased to function (or if you've uninstalled it), this freebie from the same stable (which is also 'run on demand' only) is just as good:
https://www.malwarebytes.com/adwcleaner/