This is a pretty common question on the newsgroups, and also came up on e-mail this morning - how do you change the Windows XP Embedded boot screen/logo ?
From Sysinternals.com - modify your boot.ini as follows.
/BOOTLOGO Use this switch to have Windows XP or Windows Server 2003 display an installable splash screen instead of the standard splash screen. First, create a 16-color (any 16 colors) 640x480 bitmap and save it in the Windows directory with the name Boot.bmp. Then add "/bootlogo /noguiboot" to the boot.ini selection.
There are a ton of other boot.ini flags listed on the Sysinternals site.