You should always ensure that your computer is as up to date as possible. This includes not only your anti-virus (AV) program, but also other applications such as MS Office and Adobe as well. We need to remember that, so long as your machine is connected to the Internet, it is continually being bombarded with traffic, some of which you have solicited (requested when asking for a web page), and some which you haven't. In most cases, the unsolicited traffic will be blocked, and you won't be aware of its existence. However, no anti-malware program is totally foolproof, so you still need to ensure that your defences are as secure as possible. Even if your AV is up to date, malware is being written all the time and it is quite possible that it is designed to target particular applications, such as Adobe Reader, etc, and if your AV doesn't yet have the definitions to detect it, then it will just pass through and infect the target program through code exploits and other techniques. By ensuring that applications are kept up to date, these code loopholes are kept to an absolute minimum.