Microsoft bought out Virtual PC, I predict that Longhorn won't have backwards compatability but for $350USD extra you can buy a copy of Virtual PC and XP Home Edition. Microsoft isn't so stupid, they will sell the backwards compatability with a copy of the older OS to go with it. Not only that, but OS updates will be available on the subscription level only for Longhorn. So if any worms do come out for it, either pay up or be SOL.