A very nice-looking icon set, although I personally prefer the default- that might be a case of what I'm used to, though
One thing you may want to look into: On Win XP, the tray icon didn't show for me, although I suspect that's because I'm using a display theme that has black title and task bars. It would help, though, to have a white border on that icon.
Another thing: The splash screen is broken, as in the status text Calibre displays writes on the image. I think you need to put a little blank space in there.