Just use a free antivirus service. There's no perfect anti-virus solution, but a good free antivirus program will generally keep you just as safe as the paid ones, the only benefit to paid ones are extra features which you likely don't even use.
I use Microsoft Security Essentials (free) and it's worked well. I've used AVG and Avast in the past but MSE has been my personal favorite since it uses minimal resources and works fine (AVG and Avast have gotten a bit bloated in the past few years).
To add to that, I use MalwareBytes Premium ($25 one time fee for a lifetime license). There is a free version but the paid version has real time protection, which is nice.