Hi Jules,
If pretty much all you want to do is run Powerpoint, then you may as well stick with Vista. In my opinion, you get a faster, more responsive machine by putting XP on it, but you don’t sound like you’ll be doing anything that’d need a faster, more responsive machine. We don’t support Vista on our network, so it had to go. If you’re not on a corporate network then it shouldn’t matter to you.
Having said that, so long as you follow the instructions I’ve provided and linked to, anyone can do it. You just need confidence 🙂
I doubt very much the Tech Guys at Purple World would put XP on it for you – certainly not at a reasonable price, anyway.