Good luck on that one.
For starters, many people do not trust putting their personal files (which an OS encompasses) online. Secondly, millions of people around the world have extensive know-how and have been habbitualized using Windows so I doubt a new OS will make people switch. Thirdly, most of the things Google offers is extremely basic (word doc, spread sheet, office suite), I doubt it will make any impact. Ever heard of Openoffice? It's free and it has many features as Office, or WordPerfect? These tools have as many features, if not more than MS Office, why haven't any of them even sucked a small % out of the Office marketshare? It's simple because of the reasons I pointed out above.
Lastly, the only thing Google is ever going to be good at is Adwords/Search and general Web 2.0 apps which everyone is impressed by, but won't ever really utilize in a real business infrastructure.
As far as I see it, Google is going to have such a hard time tapping into the Microsoft dominate products (office, windows), just as hard as how Microsoft is trying to tap into Google money making products (Adwords, Search)