I cannot accurately vote because I use both, XP and Vista.
I wish I didn't have to keep upgrading my os to run new software. If I had my way I'd be using my old 386sx processor with an emulated math co-processor running dos and win95. Ah, back when a game had full control over the computer without the os causing the fps to be erratic...
On a side note C2 crashes when it closes on xp after adding a sprite or loading a capx with a sprite. It's non-fatal since it happens after the project is saved but it still is a slight nuisance.