If your ISP is Comcast/Xfinity just get the Norton subscription(s) they offer for free due to you being one of their customers.
Otherwise here's a recommendation from everybody whose ever made viruses: MalwareBytes
It has a free & a premium version, you're looking for a free one so go with the free version.
(There's also Kaspersky but its a trial, so take that as you will)
Also Avast & AVG are the shittiest free AVs that work with Win7 onward, I would mention the WinXP & Vista shit ones but they are irrelevant due to them only working on the respected OS they were made for.