Create a 320x400 bitmap in the root directory and name it LOGO.SYS
You can use LOGOW.SYS file in the Windows directory as a starter.
There are many system file that constitutes Lofoff screen.
They are actually bitmaps 320x400 that just have a different extension
The hidden file in the root directory LOGO.SYS is the startup logo.
There are two files in the Windows directory.
LOGOW.SYS is the Wait while Shutting down ... screen.
LOGOS.SYS is the You may now shut-off or Reboot screen.
Make two new image files of your chice in Paint and name it as LOGOW.SYS and LOGOS.SYS and replce the actual windows file by this two.
But make sure they should be of the same size.