I was poundering the same thing as you this evening. I discovered that the thing contributing more to the look of fonts is not the cleartype smoothing, but font smoothing in general. Clear type is just one way to do it. If XP is in its default state and nobody turned off the font smoothing option in advanced system settings, then you are OK. Most people do it when they choose performance there and that switches off all visual stuff, font smoothing included.
So in my opinion most XP users will see smooth fonts. Those that don't, choose so themselves. Maybe they don't know they did, but you cannot do nothing about it. Except maybe try to educate them ;)