[cwa107]

1. Running Windows on a Mac is just like running Windows on a PC. So, your Windows installation is certainly susceptible to viruses.

2. Yes and no. If you run Windows in a virtual machine (i.e. Parallels or VMWare) and you give the virtual machine access to your Mac's hard drive, there is a potential for a virus to damage your Mac's files. Likewise, if you run Windows in Boot Camp and you've installed a driver that allows the Windows installation to access your Mac's partition, the same thing could potentially happen. In either case, this would require a very specific set of circumstances and is rather unlikely. In real world usage, any virus infections would be relatively confined to the Windows installation and not harm Mac OS X in the slightest.

3. No. Just as you can't run Windows software on Mac OS, you can't execute a Windows virus and have it effect OS X. While you could potentially download a virus, the virus would not be able to run in Mac OS X. With that said, you could potentially forward on a Windows virus to another Windows machine, but your Mac OS install would remain undamaged.