The ad, explained
Here's what the ad means:
Bill has bought from the same shoe store for years (i.e. used Windows) because he is comfortable with it. In this instance, the shop assistant is giving him bad advice (like the employees of PC World) so he isn't getting the shows that fit him (OS). Jerry steps in and finds a better fitting pair of shoes (er, Vista?), proving that the store still sells good shoes but that the assistant is crap. So basically, PC stores sold Vista on unsuitable machines, so it seemed crap when it wasn't.
Okay, this premise does seem a bit silly, since MS were at fault for getting Vista on unsuitable machines, but on the right machine it does work really well. Maybe MS should get in on the overpriced-but-works market.
It will work well in the US, since the Apple ads here feature an old balding fat guy as the PC, and a sharp fashionable young guy as the Mac (more of a contrast than the UK ads). In comparison, Gates comes across as an ordinary guy buying shoes on his own in a discount shoe store, and people can relate to that (they voted for Bush for that reason). 80% of people don't care about being cool, they just want something they can use.