The ad that made me start using Macs wasn't an Apple ad at all. Back in 1989 when I was a physics major, I came across an ad for this amazing software called Theorist in a Physics Today magazine. I had never seen anything like it before and it only ran on Macintoshes.
Well, that was all it took to get me hooked.
This is a 1990 version of the ad from a MacWeek magazine...
http://www.shawcomputing.net/racerx/theorist.jpg
And this is what it looks like running on my PowerBook Duo 2300c...
http://www.shawcomputing.net/racerx/screenshots/duo-theorist.jpg
Not an ad but the tv movie Pirates of Silicon Valley back in 99. I wouldn't say it switched me initially but it planted the seed.
Well, I wouldn't say that I made a platform switch, but I did go from being rather indifferent about computers in general to loving Macs once I saw what could be done on them without having to learn a programing language (and yes, learning Mathematica was considered learning a programing language back then).Do adverts really influence people THAT much? I find it hard to believe that an advert, how ever well produced, would make one switch to a new computing platform, which is a pretty big decision. I suppose they must or Apple wouldn't spend so much on marketing.