I think you are all not thinking long term enough. Imagine a Valve branded Linux box with Steam, Chrome and Open Office pre-loaded. Then imagine Valve either pressuring more developers to convert their game to run on this box, or helping with the conversion themselves. What would most people even need MS Windows for at home then?
This could compete with the Xbox, PS3 and Apple. If they get rolling with that, imagine how much money they could make.
Gaming has always been important to Microsoft, not necessarily hardcore gaming, but Solitaire, Minesweeper, Freecell and other goofy time wasters like that. Plus they always wanted some higher level games on their system. This is one reason that I believe MS crushed OS/2, as regular consumers are just not going to buy computers en masse unless they can goof around on them.
The only thing that could stop Valve from producing a Linux Box like this (eventually) is for MS to basically change their O/S to be Valve friendly, or just buy Valve outright.