Defensive
It's pretty unusual to see a gorilla in a space -- Windows with about a 93% share -- responding so pointedly to a 7% competitor. Seems unprecedented, or at least I can't think of any examples in any other industries. Imagine Budweiser going after Rolling Rock, only worse.
Interpretation No. 1: Microsoft can brook no threat to their hegemony. Their only real competitive advantage is their ubiquity, and if the Mac OS hits a critical mass you've got the dreaded, emperor-has-no-clothes scenario. So they've got to stop the Mac OS. And Microsoft either sucks at everything else they've attempted or, as with Office, inherits market power from the dominance of the operating environment.
Interpretation No. 2: They have research that shows that things are much worse than anyone suspects. Those share numbers above probably got a big bump from mass Vista upgrades, or they don't reflect a cratered college market, or they've got a defection model that says something like "28% of non-Mac iPhone customers are switching to the Mac OS after 9 months," etc.
Interpretation No. 3: The Hodgman/Long ads are simply pissing them off, so they've got this executive-level emotional response they're projecting through their agency.
Interpretation No. 4: They've run out of ideas. They can't talk about the product, because everyone's eyes glaze over. They tried the Seinfeld thing, which was weird. They tried to do hip -- like the Zune's "Join the Social" -- and just plain can't pull it off. So they try it again, playing off the Mac's "I'm a PC" ad. Pretty good execution, but off a lousy strategy.
This is fun.