According to The Inquirer Microsoft will release DirectX 9.0 L for Windows XP. This will basically be a renamed DirectX 10 for Windows XP.
It will make DirectX 10 games to work on Windows XP. And games such as the upcoming Crysis won't work on the existing DirectX 9.0 c. they need a DirectX 9.0 L
One of the biggest issues is the fact that Nvidia or ATI won't have any mainstream or entry-level cards until at least mid- to end of Q1 2007. This suggests that if Vista tips up around the beginning of the year, gamers will be turned off by it.