The people who make their decisions based on what Dell defaults to are going to end up with Vista - because that is their only option now. There was a time when they had a choice between XP or Vista with a new computer, now you have a choice between Vista or no computer.
So this campaign is aimed at the people who actually make an informed decision and influence large purchases - basically IT guys since businesses are the ones still using XP/2000. Insulting their intelligence is not an effective strategy.