Open any Explorer window. Click on option in the VIEW menu.
Choose see all files on teh VIEW subsection of the OPTION box.
Look for the files. There are three of them:
logo.sys in the root directory. The root Directory is what you get when you open first "MY COMPUTER" and then the hard drive (normally the C:\ drive). That's the boot up screen.
logos.sys in the WINDOWS directory. That's the shutdown screen.
logow.sys also in the WINDOWS directory. That's the screen which appears during shutdown.
Back up the files. This step is not necessary, but I really recommend it.
Open the files with any image software that supports windows bitmaps. You'll at least have Paint.
Change the files and resave. Remember that the image size of the files is different of that of your screen so they will be horizontally streched.
Now you might want to change back to viewing just the necessary files. Again in the OPTIONS box of the VIEW menu of explorer, click on Hide the following files: (I not too sure on whether this is the literal name of the option. If it is not it is something like that).