A previous poster hit the nail on the head with these and exactly what i think when I watch this.
99.5% of PC users have only ever used a PC, they don't know of anything else. So of course they are PC's, why Microsoft feel the need to preach to an audience that already use PC's I don't know, the adverts don't even tell you anything about PC's, what's good about them, why these people use PC's instead of .... instead of what? ( that's the reaction of the majority of people )
I've yet to find a Mac user who hasn't or doesn't use windows. I use XP when I have to. Plenty use it at work etc but we choose to use OSX because we prefer it. Rather than just using it because that's all we know so these I'm a PC adverts will do nothing to switch mac users to a PC.... and that's the only conceivable target I can think these ad's were made for baring making Microsoft feel better about themselves.
The I'm a mac adverts work because they stretch out and define what a mac is to PC users who don't know what a mac is and what it does. They may be stereotypical but it works to define the brand while giving people information on the difference and benefits to switching to a mac.
The microsoft ad's confirm to me that Bill gates uses a mac, that some guy with a headband uses a PC, some guy who wears jeans and that fatality uses a PC to game on....
They should have focused on the benefits of PC's, what PC's can do, how easy they can do it, the features of the OS over the likes of OSX or XP.......I can see why they came up with what they did.