Microsoft know that in the long term the role of the operating system might dwindle since a lot of the stuff people do is moving onto the web. Salesforce, zoho and google apps are examples of this. This is extremely threatening to them since their main competitive adantage lies in their desktop dominance. So they obviously want to dominate whatever they think might be the next big thing, and one of these happens to be search. And the 500 pound gorilla in that market happens to be Google. And the fact that Google is also trying their hand at an online suite that might replace offfice one day makes them an even bigger threat.

Note that I'm not saying that the desktop is obsolete, or that the one that controls search controls the world. I'm merely saying that Microsoft is looking at current trends which point towards a lot of data and functionality moving to the web, and are hedging their bets accordingly. As they should.