It can't be changed using registry. You'll need to edit "%windir%\System32\sysdm.cpl" file in Resource Hacker to change that logo. You can visit following tutorial which can teach you the complete process:
VG a question, in ur site abt the make VXP like VIsta widout any trans pack, I've downloaded and used ur theme, but it just makes my XP luk like a Win 98, not like a Vista