My preferred Antivirus at present is Avast. It is free, just be sure to opt for the free version. E-mail registration needed but you don't get any junk mail from them. I have removed AVG from three computers recently as it was slowing them down noticeably. Windows firewall is OK and if you are connecting through a router, check it is using NAT and this provides an extra level of security.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been