I use...
I like the idea of advertising the Apple store (though I think you'd need to be careful about making it out to be a place kids can hang out on days off to play online.)
I think a good commercial can't succeed, though, without letting people know that they can use Microsoft Office on a Mac. Of course, you can't do a commercial just on this (I'm sure MS wouldn't agree to it), but what about something close to an antidepressant commercial:
"I use mine [iMac] to create movies with iMovie."
(two second screen shot dragging around a clip)
(next person)
"I use mine to do work at home with Microsoft Office."
(screen shot of the office "Memo" template, which everyone will recognize, and something in Excel)
(next person)
"I use mine to organize all of my digital photos with iPhoto"
(screen shot)
(next person)
"I use mine to organize my life, my work, my kids' lives, and *his* life with iCal"
(screen shot of selecting and deselecting the several calendars in iCal, syncing it to a Palm, and walking off with the Palm.)
I don't like the idea of bashing Windows as far as viruses, etc. And, as much as I hate to say it, XP never crashes on me (at work - I don't allow Wintels in my house). Just say, "the safest, most stable operating system available" and be done with it.