I'd be kinda surprised if you could just install 10.0 over 10.1 and have no problems, but you never know...
I wonder if Microsoft consulted Wolfram about how to do their licensing for XP: they're both the most frustrating licensing systems.
Even worse, with the number of times Win 98 crashed on me and I had to redo Mathematica, I'd hate to think what the poor people dealing with XP have to look forward to.
-Rob