Hmm... licensing...
Volume license plans? A nonprofit organization could note MS and ask them for an offer on Windows Fundamentals. Or MS should offer one. When Ubuntu becomes usable for everything including Winamp in a seamless environment (Automated WinE), I'll jump from
XP. (Too bad Vista arrived.

)
Linux in general, still feels REALLY limiting in terms of what you can do even when it offers a lot app-wise. As an artist-PC user I can say it really frustrates me to use open source variants of stuff I paid (Who still uses GIMP nowadays?), OR go through WinE (ugh).
It'll be getting much more use in the near future, but hands down, closed-source alternatives win by having much better featured, better supported software that Linux users won't even consider buying because they can't afford to (mostly).