It's true - Mac viruses really do exist, although fewer viruses target the Mac OS compared to Windows. In addition, there are cross-platform threats that can impact Mac users. Though the risk of cross-platform threats is relatively small, it's a risk that is on the increase as attackers move to Java and other third-party software exploits. Fortunately, there are several excellent Mac antivirus offerings - and some are even free.
Sophos Anti-Virus for Mac Home Edition offers business-grade protection free for home use. And the protection literally is business-grade - the free version is based on the same Mac virus scanner Sophos uses in their Endpoint Security line. Best of all, Sophos Anti-Virus for Mac Home Edition detects both Mac and Windows malware. For example, if you use Parallels with a Windows virtual machine on your Mac, files in shared folders on your Mac will be scanned for both types of threats.
Norton Internet Security 3.0 for Macintosh supports the PowerPC G3/G4/G5 processor only, running Mac OS X versions 10.1.5 through 10.3.9 only. At about $70 USD, Norton Internet Security 3.0 for Macintosh may be a bit pricey for such older versions. On the flip side, there aren't too many Mac antivirus scanners that support those older versions, thus Norton Internet Security 3.0 for Macintosh may be your best bet.