It's true that there are no viruses for OS X, but I was recently updating a Mac to Tiger and installed a copy of Norton (we get it for free through company access anyway) on it just as a precautionary measure, and it found 13 viruses! They were PC viruses, so there was no risk of infecting the Mac, but keep in mind that every time you're sharing files with another computer, you're putting other computers (PCs) at risk. So, you can be selfish and say there are no viruses for your OS, or you can protect others.