Stardock has released release candidate 2 of its freeware boot screen changer. RC2 displays the progress animation more smoothly and includes a full screen preview of what a boot screen looks like before applying (complete with animated progress animation).
BootSkin supports Windows NT, Windows 2000, Windows XP, and even Longhorn. It is the only boot screen changer that doesn"t patch the Windows kernel to change the boot screen (making it much safer to use).
Windows boot screens are 640x480 with 16 colors but with tools such as SkinStudio you can optimize those images so that they look pretty good.