All the programs i mentioned are free.
Microsoft Security Essentials is antivirus from Microsoft, it can be installed from the microsoft website and it works really well with a low footprint on resources.
Malwarbytes and Spybot Search and Destroy are two other programs designed to target Adware and Malware, very common stuff, easier to remove, but you get these more often than "viruses".
Running all three in combination is a very solid set up, and uses few resources. Malwarebytes can monitor your web sites to alert you (like a firewall, but actually effective and wont mess with other programs).
Norton and Symantec are heavy on system resources, a bit outdated, and not a good choice compared to the others available. Most antivirus companies have admitted they have lost the game, and viruses have the edge over the antivirus type softwares.
what im saying ultimately is a lot of your computers safety is going to rely on you the user now, and you cant trust software to keep your computer safe. After i visit a fishy website, say for streaming TV shows, or whatever, iwith lots of ads and pop-ups, i know damn well to run Malwarebytes, Spybot, and Microsoft Security Essentials after im done. The firewall simply isnt enough anymore, it takes active thought on your part to keep the rig running to spec. If you feel something running slower, or acting weird, its never a bad idea to run your programs (malwayre, spybot, MSE) to see whats going on. Keep your definitions updated before each scan too.
you could also set the programs to run automatically on a schedule, but i dont like scheduled services. I prefer to launch the program myself and do it