others seem to have it pretty much covered...
but this is how it works.
if you want to "downgrade" to XP you put the XP CD in the computer, start the computer from the CD and during the XP installation delete the partition that vista is on and install XP onto a blank, newly formed, partition.
Then when you come to install Vista onto another machine it will install no problems but on first use it will ask you to activate it via the internet, This may or may not work, if it does great, if not no hardship.
All that happens if it doesn't activate is you will be presented with a (very long) number and a phone number, you call the number, enter the long number on the phone keypad and you will be given another long activation number, you enter this on the computer and it will activate windows for you.
the worst case scenario is going to be that you get put through to an operator, then all you do is explain the situation you have described above to them and they will give you the activation number.
As it is a retail version of vista you have you are perfectly entitled to move it around computers as long as you don't have it on more than one at a time.