Some anti-virus solutions can slow down your Mac, but to be honest, the best anit-virus app is you, the user and your brain.
- Don't visit questionable websites or website you are unsure about.
- Don't install pirated software or software downloaded from a questionable or unknown websites or untrutsted sources.
- Java is a vulnerability right now, if you do not need it, don't use it.
- Use a browser filter and pop-up blocker
- Don't open email attachments from email addresses that you do not recognize.
- Install security updates when they become available
- Educate yourself as to what threats are common and active.
- In effect, use your own brain as the antivirus filter.
Follw that advise and in MOST cases, you will be fine and won't feel the need to have to install antivirus software.
Do NOT waste money on commercial anti-virus applications, particularly not Norton which will mangle OS X.
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:
Never enable Java for a public website that carries third-party advertising.
Linc is actually not conservative enough there. Third-party advertising is not remotely the only way that Java can be used maliciously. Sites that do not have any advertising at all can and have been hacked to contain malicious Java applets.
Never enable Java in your browser, period. The only way Java can be at all safe these days is running in a virtual machine that is isolated from the main system, or on a secondary computer with no important data.