Taken from another website, untested so i'm not sure if it will work, and as far as i know anything over 1024 x XXX wont work.
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The boot screen in Windows Vista has easy to change, you can choose the boot screen as you want, your photo, your girl friend or your family. Just using the great tool by Dan Smith it’s possible to automatically generate the replacement language MUI file that contains your boot image files. Just prepare two photos that have resolution 800 x 600 and 1024 x 768 and saved as 24-bit windows bitmaps. And follow these steps :
1. Download Dan Smith’s Vista Boot Logo Generator. And Install it !
2. Run the software from the start menu.
3. Hit Browse for image and select both the 800×600 and 1024×768 resolution bitmap files for your boot screen.
4. Click on File and select Save Boot Screen File as and save your file.
5. Next, you will need to replace the winload.exe.mui file in C:\Windows\System32\en-us with the one you just created. Before you copy in your new file, make a backup of the original. You will need to take ownership of all files in en-us as well as give your account file permissions to replace the file as well in order to copy the new winload.exe.mui file in.
If you already finish these simple steps, now reboot your Windows and you should see your new boot screen. If your windows booting problems. Please boot up with the Windows Vista Installation CD and replace the winload.exe.mui file with your backup.
