Upandaway,
I think you will find (certainly with previous versions of Windows ) that you were permitted to install it on multiple machines but they couldn't used simultaneously.
This could answer your "Can I install it on my home system and my laptop?" question. However, if your downroute using the laptop and Mrs. Upandaway is at home using the desktop then this is classed as a breach of the licencing agreement.
They would have a pretty hard time proving it unless their is more to XP than MS are letting on, which would not be unheardof from our friends in Redmond.
Whooaahh!!!