There is a reason why Apple OS like Leiopard, Snow Leopard is considered virtually virus free. This is because the registry in Apple works a little bit different than in windows. In Windows we have a single uniform registry which stores every aspect of our computing. So when a virus attacks a Windows PC affecting the registry the entire System is affected.
But in Apple OS things works differently.They have what we call a segmented registry. That is, individual registry for separate applications. So when a virus attacks a certain Application affecting their registry it is contaminated only within that application registry and the whole system is unaffected. This is a brillant piece of computer science from Steve Jobs and explains why Apple OS is so fast and much more optimized than Windows.