One thing to check when downloading free "protection" is if the product provides active protection. Several free or trial versions are scan or on-demand only; they don't monitor incoming data.
Avast monitors in real time , & almost always scores rather highly on sites such as AV-Test & AV Comparatives . Avira is another free antivirus that monitors in real time . Comodo has a free security suite (Antivirus & Firewall) that monitor in real time . Comodo has the best free firewall available , bar none , & is better than most paid firewalls . Comodo is more for an advanced user , but novices can figure it out with a bit of time on their side . I am one of those that loathe anything Symantec (Norton). Zone Alarm has a free security suite that monitors in real time . The antivirus part is powered by Kaspersky . In the end though , a computer is only as secure as the person behind the keyboard .