A good anti-virus is the very first thing that you should install on a new computer... before any other programs get installed.


For Apple Macs, Sophos Free edition is a good basic protection package:



For Windows XP, Vista & 7 PCs, Microsoft Security Essentials is a good basic protection package:

  • http://windows.microsoft.com/en-au/windows/security-essentials-download

In Windows 8, it's already installed on your PC... renamed as "Windows Defender".