The thing is that I'm not a fan of having an antivirus in Windows nor having a firewall, they need lots of resources, makes PC slowlier and clumsier... 
You can always turn off real-time protection on your anti-virus scanner and just do a scan on every downloaded file and a full scan every 1-2 weeks. While this is significantly less safe, it's a lot better than no anti-virus and won't use up much resources. Anti-virus should really be a failsafe anyways, you shouldn't solely rely of them to keep your PC safe.
Windows 7+ has a built-in firewall which doesn't use much system resources.