Excel Licensing?

I have the Student/Teacher version of Office, licensed for 3 machines.
And it's installed on three machines and all is well with the world.
Almost...

My wife was upset with the performance of Vista on her computer so I
installed XP on it in a dual boot arrangement. So my question is
this. If I install Excel on her XP partition, as well as the Vista,
does Microsoft still count that as one machine or two?

Or alternatively, can I execute the Excel installed with Vista from
XP? XP has access to the Vista disk, but obviously the XP registry
won't have various Excel entries in it.

Has anyone faced this question? Thanks.

Bill