I'm working with Macs and PeeCee's at my office, 5 Macs for 10 Peecee's (snif). I just can tell you something for the past two month, I've received +/- 10 viruses on the PC's and Mac's, they were all "designed" for Windoze plarform. two of them bypassed Norton and have infected a machine.
But: All the Pc's are protected and live update are made constantly, and they still get infected...
For the Macs, yes I got Norton, in the bookcase, never used it, and never got a problem, the only viruses that have ran on my mac were trough virtual PC to see what was going to happend. (just for the pleasure to see Windobe crashes... he he he).
There's only one thing to do with viruses: if you don't know what an attached file is on your e-mail, do not touch it and ask, mostly if you don't know the sender of the mail.
If you have doubt about a file or an alert, you can dtill go to the norton site and check in the Virus encyclopedia.
But don't worry, as said upper, we are not the target for those things, we're too small... and that's exactly what's making us so powerfull.