Nothing is completely uncrashable, not Windows, a Mac, a *nix box, or whatever. But the Mac OS does seem less prone to the gradual onset of general system instability that you see with Windows after a while of use/installations/etc. And while it's true that a properly configured and maintained Windows installation can be just as stable, virus-free, and secure as OS X, it also takes significantly greater effort and knowledge to achieve this on Windows.