First off all, your Internet Service Provider may provide free Anti-Virus, for example Comcast uses Norton; the current version is effective and is no longer a system-hog. One caveat, you may need to install something else first to get it, in their case something called "ConstantGuard," which is a roach-ridden piece of dungware. Fortunately, after installing Norton, you can then uninstall the ConstantGuard.
Otherwise, if you're looking for something free, I recently switched from AVG (which isn't bad) to Avast! (which reviews found to be a little better).
In either case, also install MalwareBytes, which can stop other kinds of malware that virus scanners may miss. It too has a free version.