To turn this away from a Windows bashing forum, since many of the monks do use windows enviroments. (I do, but prefer Linux). XP is a concept here, not a product. Plus in the many roleplaying games I've been involved with xp was often the shorthand.
Seems there are some prior tradmarks:
Super Soaker XP-90
Interesting line from XP draft
"This is the first W3C Working Draft of the XML Protocol (XP) requirements"
A RIP Server, and so on.
How many uses of XP are out there? They can go ahead and say "Windows XP" is a trademark, but can they go after past trademarks that use XP?