The operating system IS the operating system…
Eric, over at Shoulders of Midgets has a very nice little piece on what is, and what isn’t an operating system. For those who want the short version: it’s not Word. 
It reminds me of what someone said in response to Sun’s “The Network Is The Computer” slogan: “The computer is the computer; the network is the wiry spaghetti-like bit that comes out the back. Apologies for any confusion caused…” (anyone know who, by the way?)