RE: Dell and the install disk.
I have used an old Dell XP disk to install XP on an XPS 1330 (very recent vintage, shipped with Vista), with no problem. It doesn't ask for a serial number. I've also used the same disk to install XP on an old Dell Optiplex GX110 (shipped pre-XP), with no problem. Again, no serial number was asked for.
Vista may be a bit different, but I installed Vista Ultimate on the same XPS 1330, when it was actually licensed for Vista Home. Again, no serial was asked for, and I got no errors in over six months of running it.
As for the legality of all of this, I believe that you can downgrade a system from Vista to XP without any issues. Dell has paid MS for a license already; how you use that license doesn't matter (IIRC). BUT upgrades are almost certainly a violation of the license agreement. There does not seem to be any mechanism for enforcing that agreement, however.
Woody