Would I lose XP license if I do full installation of Vista?

Would I lose XP license if I do full installation of Vista? My friend told me about it. he also said same thing about upgrading xp to vista. for example, if my Vista installation goes bad, I want to reinstall XP again on HD, would my key become useless even though I activate it over the phone.

Somewhat related question

I had XP Pro (actually an Academic version) on my PC and did a clean install of Vista Business (using the double install method (upgrade on top of a trial version of Vista!) using an upgrade disk). I haven't yet activated since I'm waiting for the 64 bit media in the mail. Also I assume I will just be able to do the same double install of the 64 bit version OK since I haven't yet activated

So my question is: Can I effectively now reuse my old XP Pro on a different machine (maybe one I'm about to build)?. I would expect I would have to call MS as the hardware will have changed but they wont have any link to the fact that the old version used to be on the other PC that was now got Vista on it?