I recommend AVG, Avira or Avast (only one) as anti virus. I would use the Windows 7 firewall. I also use Spywareblaster to deter malware, and both Malawarebytes and Superantispyware to scan and remove threats. All these are available with free downloads. I also use free CCleaner to clear history, cookies etc and to do a light registry clean. Dont forget to remove your existing a/v.
OnePlus 5 review