I installed the new iTunes 9 update tonight, and started getting an error every time iTunes then started. The error was in a window titled QuickTime Handler Window, and the error said that the application could not start because "d3dx9_36.dll was not found".
The error seems to be that a component of DirectX is not present, or up to date, and the fix is to download and reinstall the DirectX components. There's a walkthrough on about.com that explains what to do.
I tried to log on to Vista this evening and got this message for no obvious reason. Nikki's profile was fine, it was just mine that was broken.
I followed the fix suggested in a post at technologyquestions.com that involved a fairly simple registry fix to re-instate my profile that for some reason had been turned in to a back-up.