well depending on the manufacturer most push an f key or the esc key to change boot order/options a few seconds into startup, (usually a black screen.)
and seeing as you don't even know how to do most of this simple stuff for installing an OS, may i ask, have you partitioned your HDD for this? i wouldn't advise installing Vista over XP yet, as even with build 5600 (RC1) Vista isn't quite stable enough to run as your main(and in most cases lately i've seen + your only+) OS. 5600 is much more stable than the others before it and it's operation is quite smooth compared to the 5536 and earlier builds. So using your judgement, if you don't care about the risk of vista corrupting your HDD, or at the very worst, bricking it (I've seen it happen.) install Vista over your XP as your primary OS, if you do care, research into partitioning your HDD to use Vista as a novelty OS, until it becomes fully public, and "marketably" stable.
