BOAC,
If you have access to XP installation media (and a license), then you can presumably buy what you want, irrespective of what pre-installed OS there is on it.
Frankly, I wouldn't bother backing up Vista before trashing it and installing either XP or Ubuntu.
Anything for familial harmony!
Oh yes - if you have got an XP installation CD, make SEVERAL backup copies!!
I have 2 backup copies of mine, just in case.
SD
ps - I think that your son and I would get along very well! What an excellent summary of Vista.