Considering there are no viruses for OS X you can be sure there are no viruses. If you update your software to 10.6.8 (http://support.apple.com/kb/DL1399) you will have the most up to date system. OS X is secure however if you have somehow installed a Trojan (totally different than a virus) which requires the user to consciously download and install the malware the 10.6.8 update will take care of it for you. Keep your software up-to-date using the Software Update tool. Also prior to doing ANY OS X upgrade or update backup your system and I recommend disconnecting any peripherals (except the keyboard and mouse) when installing. Once the installation is complete then re-attach any peripherals.
Also ignore the prior poster's advice about getting antivirus software, he is mis-informed. Absolutely no antivirus or anti malware software is needed for OS X. They frequently cause serious problems when installed and are only recommended by the software vendor itself. Installing any anti malware software on a Mac is a waste of time, system resources and possibly money!
Odds are heavily stacked against your Mac having a virus, since none currently exist.
There are other forms of malware that can infect a Mac such as Trojans. The best tool to prevent them is located between your ears. The reason is that a Trojan must trick you into allowing it to be installed on your Mac. So if you are careful to not allow the installation of suspect software, there is no need to worry about them. In addition, most AV software is best at detecting virus hence the anti-virus in their names. They are not nearly so good at detecting Trojans.
Do not install any commercial "anti-virus" software. It's all useless and can only cause problems.
The Mac OS has built-in detection of Mac malware in downloaded files. The recognition database is updated daily. Although the protection isn't perfect, it's as good as you'll get.
If you need to able to detect Windows malware in your files, use ClamXav -- nothing else.