the reason why there are so many virii out for the PC is because there is a larger user base. It would be kind of pointless for a "hacker" to write a virus for an OS that only a small amount of people use.

Personally I love Windows and everything it can do, but I also enjoy Linux. Its been years since i was on a Mac OS, so i dont know everything thats new to them, but i don't care, because they arent necessary for what I do.

edit: these commercials kind of piss me off, because if the user of a windows computer knows how to take care of it, it wont reboot, get a virus, etc. (all that has been mentioned already). My normal uptime is 1 week for my PC, and thats because i have to reboot it for installing/uininstalling programs/drivers/etc.