i did consider that. i even bought the hard drive to do it with (which is now in the machine and just full of data instead).
i didn't do it because i still wasn't confident about what could happen. the problem is, currently, xp is installed across two drives that are in raid 0. so they aren't really convenient to swap out. i did consider just removing their cables but that isn't convenient either as i'll have to keep going inside the machine to do it. it also means that i'd be losing the advantage of having raid 0 when they are disconnected - maybe that would be hardly noticeable.
so, i thought, i could perhaps leave the cables connected and just disable things in the bios. i've had machines in the past where i'd dual boot between two drives by disabling one or the other in the bios. BUT, it's vista again, and i don't know how happy it would be if i tried this sort of dual boot disabling a drive arrangement.
i say that as i know i've had xp see drives that were disabled in the bios. i can't remember when it was and what happened but i distinctly recall wondering how it could do that - i think i used it to to my advantage by accessing files that were on the disabled drive. (okay, maybe what i described was something more to do with the hardware at the time rather than xp being too clever - or i just don't get how a bios really works, so it doesn't really disable things when it says it has).
anyhow, it's vista, so i'm inclined to think if xp could do that, then vista will probably do something even more weird, i.e. it will try and take over the drive that is meant to be disabled in the bios, and it will mess things up somehow.
if i wasn't using the raid 0 drive(s) then i'd do the simple swap over of a drive with xp and a drive with vista. but it really isn't practical right now. i've been using this xp installation for 3 years without a reinstall so i really don't want to give it up yet as it works perfectly - so that means keeping the raid 0 setup.
if anyone knows for sure that i can disable the raid drive (or disconnect them) and then install vista on it's own drive and then reconnect everything and then dual boot using the bios method then i'll do that.
in other words, if vista isn't going to eat my raid drive as soon as it sees it then i'll try it. i'd prefer to do this hard drive swap method rather than install vista on a second partition, i'm just suspicious it won't be that simple.