I run Windows, and two flavors of Linux on my Mac, all at the same time under Mac X, using
Parallels.
Never had a virus or anything under Mac X or Linux but my Windows install seems to get hosed pretty regularly. I don't worry about it though, don't bother with the hassle of running virus software, since Windows is just a file on my hard drive. With my setup if something bad happens with my Windows install I just throw away the file, grab the backup file, and I'm back up and running in less then one minute.