That's sort of vague. Linux and Windows vast differently from one another. One is a clone of the *nix OS and Windows well, is Windows.
The main reason though Linux is more secure is the way user accounts and permissions are setup. A virus in Windows doesn't have the same affect that it does in Linux as an example.
Every computer is vulnerable though in a sense of security, it really boils down to the administrator of the machine making it more secure.