You posted your question in the Leopard forum, but assuming you are using Snow Leopard as in your profile information, read: Mac OS X Snow Leopard and malware detection
"Internet security" exists mostly between your ears. Don't download torrents, don't fall for phishing scams, and never supply your user name, passwords, or any other personal information unless you know who is asking and how that information will be used. Know what you install on your Mac, what it is supposed to do, and how to get rid of it when you don't want it any more.
Understand that every software product, without exception, that claims to "speed up", "clean up", "optimize", or otherwise perform some magical incantation that pledges to make an older Mac perform better than was possible when it was designed is 100% fraudulent and guaranteed to ruin your day.
Maintain backups, test your backup strategy, and prepare for the inevitable and unpredictable event that will cause your hard disk to crash and all the information on it irretrievably lost.
No product, nothing you can buy or install, will ever substitute for these guidelines.
One more thing - disable Java in every browser you use. Java is a platform-independent programming language that Apple does not control, and remains a potential vector for malware intrusion. Most websites do not require its use, but if you choose to use Java, understand the risks involved in doing so.
You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.
More useful information can also be found here: