It's an anti-piracy initiative. Here's Stephen explaining the need for a "diversity of revenue streams":
There will be ad-based revenue streams. There's an opportunity to draw those pirate customers into the revenue stream. We want to draw them into the Windows family and maybe there's an upsell opportunity later.
Joe makes a secondary point that's an intriguing idea — he credits not only the economy and netbooks, but Microsoft's "I'm a PC" ad campaign for putting Apple on the defensive.
We're not sure if the ads below are making a bigger difference than price in this economy. But yes, marketing matters. And yes, the "I'm a PC" ads are better than those weird Jerry Seinfeld commercials.