How about:
Windows EW (for Even Worse) The slogan could be "Bet you thought we couldn't do it, but here is is!)
or maybe:
Windows LE (for Luser's Edition) "If you thought that XP was good, then this is the perfect OS for you!"
or:
Windows 4Q (and 4Q doesn't stand for 4th quarter) "It's what we do best, but now it's in our name."