I don't think you're in trouble if you get Vista, it's just not the recommended operating system. I guess it could be troublesome during exams if they don't have everything smoothed out by then. If you go to Best Buy, though, I doubt you're even going to have the option of getting XP. The computers there are all pre-configured and will be loaded up with Vista. Even people configuring their own computers are starting to have trouble getting XP, so I'd be extremely surprised if you're able to find it on an off-the-shelf model. Just make sure to get (or upgrade to) Vista Business.
Thanks for the advice.