You are strongly advised to have an antivirus program installed on your computer to protect you from any malicious software (malware) that your computer may come into contact with. Malware is an umbrella term that refers to any software written with malicious intent. This includes viruses, Trojan horses, spyware, adware, and ransomware amongst others.
There are both free and paid versions of antivirus software available and the choice to use one or the other is entirely yours. If you decide to use free antivirus software, make sure you get it from a trustworthy source or download it directly from the supplier's own website.
All email attachments and downloaded files should be saved and then scanned with an antivirus product before being opened.
Antivirus software is only effective if it's kept updated, as new threats and viruses are created all the time. Ensure that your product is set up to automatically download updates, ideally on a daily basis.
Some of the best antivirus options tend to be the all-in-one Internet security packages that can protect you behind a firewall, hide you from online attackers, monitor and protect your web browsing and provide antivirus features in one easy-to-use interface/product.