You should follow these best practices and guidelines when running up-to-date antivirus software. 

What is Malware?

Malicious software (malware) comes in many forms including viruses, worms, Trojans, and spyware. Malware can cause computers and networks to become overloaded, work improperly, or fail to work at all. Malware can also corrupt or destroy any or all of the information stored on an infected computer. Much of the malware produced today, however, is designed either to send unsolicited commercial email ("spam") to millions of people, or to steal individuals' confidential and personally identifiable information (names, addresses, passwords, financial and medical information, etc.) so that it can be used to create fake identities (identity theft).

Modern malware is distributed through email and web browsers, making it very difficult to avoid. An unprotected computer can be infected with malware in less than 20 minutes after connecting it to the Internet for the first time. Even when protected by a firewall, a computer with no antivirus/malware protection software installed will be infected after just a few hours of casual web browsing. Although Apple's Mac OS X is more resistant than Microsoft Windows to some types of malware, there are several malware attacks to which it is vulnerable, and more are being released all the time. The "conventional wisdom" that Macs don't need to have antivirus software installed is no longer correct.

If your new computer came with antivirus software pre-installed, it was most likely just a trial version. Unless you've visited the software manufacturer's web site to purchase a full subscription, chances are that it's no longer downloading updates (or will soon stop doing so), leaving your computer and data at risk.