You have probably read all the performance previews of Crysis, both under Windows Vista and XP.  If you haven't had a chance to check those out, the bottom line is that Windows XP performance is far better than Vista.

The "Very High" settings were Vista exclusive until someone decided to play around with .cfg files. According to the post made by "LennyRhys" user on the crysis-online.com forums, you can run the game with very high settings, even on your XP. Of course, you will get a 10 FPS drop, but it will look very close to Vista's highly praised DX10 graphics.

According to that post, all you need to do is tweak the configuration files in \CVarGroups\ by copying "very high" lines and then paste them over the "high" settings lines and the game will load the highest possible settings. As you can see from the picture below, you get better shadows, the game looks more realistic and the water looks much more natural.

So there you have it. Crytek likes Vista more than XP. You can find more info here, and here are some pictures to show you the difference.