Easy way to boot.ini
Ok...here's an easy way to access boot.ini [Assuming u have WinXP]
Go To Control Panel> System.
Click on the Advanced Tab.
Click "Startup And Recovery" Settings button.
Now on the form that opens up, click "Edit" to manually edit the boot.ini file.
BTW, I don't think boot.ini holds any info to your loading screen Bitmap. It is inside ntoskrnl.exe, as LordDJ mentioned. You can use Resource Hacker to modify it.