Any game involving Duke I root for the other team.
Mostly I only "root against" players I feel are overhyped, or that I'm tired of hearing people rant and rave about. Tom Brady being a recent example. I never want anyone to get hurt or anything, but I figure if they lose, sportswriters will stop jerking them off.
I'm getting really tired of the Troy Smith Heisman thing. Good grief, the guy averages 213 passing yards a game, and for the exceptional athlete he may be, he doesn't rush for many yards, either. Yet every time Ohio State wins, people say he's just distancing himself from the pack.