> sudo rm -rf /*
Password:Not much of a virus since it asks for the root password, is it? Plus, some distributions don't even have sudo (of course they do have other means of logging in as root).
Even better, get an OS hosted in a VM. Keep personal files in another hard drive which is mounted read-only by the OS in the VM (but read/write by your primary OS). Save its state and load it every time you boot it. With such a setup you can't possibly have any viruses, but you could still be a victim of credit-card fraud, for instance. Your data isn't much safer, but your installation certainly is. I won't get into much depth but it's worth to try it if you're interested in running Windows XP or something without getting any viruses.