All computers running a Windows Operating System (Vista/7/8.1/10) are required to have an active anti-virus program installed and up-to-date. If the computer is running Windows 8.1 or 10, then an anti-virus program is already included in the Operating System so an additional one is not really needed. The included anti-virus software with Windows 8.1 and higher is called Windows Defender and you just need to make sure it is turned on.
You may purchase an paid program, such as Symantec, McAfee, WebRoot, Kaspersky, or any of the other subscription-based products. If you do not want to pay for one, you can use the included one as outlined above included in Windows 8.1/10. If you are using Windows Vista or Windows 7, you can download the free version of Windows Defender called Microsoft Security Essentials by going to the Microsoft web site at the following link: Microsoft Security Essentials
We do not require an anti-virus program to be installed on the computer running the Macintosh Operating System versions, but we highly recommend it. This is because over the past year or two, more and more infections are being written for this Operating System.