About 90% percent of the world’s desktop computers run Windows. This monoculture, a clueless Windows user base and the malware industry concentrating on Microsoft make Do I Really Need to Buy an Antivirus Program? depend on whether your computer runs a Unix/Linux or a Windows operating system. Linux/Unix systems are not necessarily more secure than Windows but free or Open Source antivirus programs can be state-of-the art protection in the Linux/Unix realm, whereas in the Windows domain this is definitively not the case as a result of the three factors outlined above.
Threat sophistication, e. g. Trojan horses, keyloggers, password stealers, identity theft and ransomware just to name a few, and the semantic game plaid by vendors of paid antivirus programs demand that you had better look for optimal (likely 360°) protection against malware. Optimal malware protection not only depends on the OS under consideration but also on how a particular computer is used. As the majority of PCs in the world are connected to the Internet and run Windows they need to have more than antivirus protection as explained in Bright Hub’s article Antivirus Programs and Antispyware Programs: Do You Really Need Both?
If someone has basic antivirus protection only and surfs the Internet she or he is acting like a tourist visiting a country where there is a cholera and pest epidemic whilst having been vaccinated against exactly one of the two. Despite that there are excellent free antivirus programs such as, for instance free AVG, no top vendor releases its 360° antimalware solution for free, and combining all sorts of free different antimalware programs to achieve 360° protection can be difficult to manage and lead to compatibility issues. As a best practice do protect your computer with 360° antimalware product and have it download the latest AV signature files every 4 hours. Also, apply the OS security patches (automatic updates) und make frequent backups.
Despite that some vendors of 360° antimalware software include free online backups you may better make traditional offline backups to an external hard disks for example for the fact that your data may be sensitive, and the time it would take restore, say 100 GB, from an online backup.