I've been using Macs since the late 1980's and have never once seen a 'real world' virus for this platform. If you're worried about potential infection of a Windows drive, then opt for the free and open source ClamAV as mentioned above.
Honestly, running applications such as Norton is a huge waste of time and resources.

---

MacBook Pro 15" | Mac OS X 10.6 • 2 GHz Intel Core Duo • 2 GB DDR2-667 • ATI Radeon Mobility X1600 128 MB • 100 GB Serial-ATA
Hackintosh | Mac OS X 10.5.6 • 3 GHz AMD Athlon 64 X2 6000+ • MSI K9N AM2 • 2 GB DDR2-667 • NVIDIA GeForce 8800 GT 512 MB • ALC888 Audio • 300 GB Serial-ATA
iPhone 3G • 8 GB