I know with windows 95/98 that changing the picture is as easy as creating a new logo.sys file(actually a .bmp), and that you are able to animate these fairly easily
I haven't doen this in a while, but heres what I recall
Create logo.bmp as a 640x480, 256color .bmp
Resize to 320x480, windows will stretch it when its used
Rename to logo.sys, and copy it to c:\
Now to animate it, uses either the first 16 colors in the palette or the last 16(sorry, can't remember), and it cycles through them
Say I made color 0 black, then color 1 a little lighter, and 2 even lighter, until I get to white as color 15, and then 16-255 could be whatever static colors I wanted. Now If I made my image all color 0, I'd have a solid black .bmp. When I loaded windows though, it would start as solid black, then any pixels that are color 0 would cycle through the 16 available colors- basically it would fade to white, and then go black and start over
Not true animation, but it could be used for flashing test(colors 0-7 black, 8-15 white) or a Williams style screen
To be sure, check google for something like "windows 98 startup logo.sys animation"
win2000 I have never messed with but I think it's similar, and XP I know there are prog's out there that can change the startup screens and sequences