What would be AWESOME would be a if you could use a gif instead of a bmp. Then you could sub in the goatse marathon gif for the boot screen. ;)
I realize that you are really against this project, but you must realize that its all fun and games as long as peeps are doing this to their friends at home and not at work. That's how hacks started the brains over at MIT started pulling pranks, then called hacks, and it was all in good fun. Of course they moved on to prank codeing and "viri", but it was still all in fun. While at CMU a friend of mine hacked my Mac!!! It was a powermac a really really old one, so that I could only save to the desktop. He also pasted a huge porn poster on my bathroom window the night of a big date. But it was all for sh!ts and giggles no one got hurt, I had my data backed up, I didn't let the girl into the bathroom. Its called fun. I realize that there are some bad people in the world but you have to realize that we are talking about a hypothetical joke, not an undetected baegle variant. I mean who didn't love leaving a floppy full of porn in your friends system when their parents were over??
Movin on up. The only way I know how to alter those types of files without an admin pass is with the win2k boot cd and recovery console. However, if the PC is on and your friend left it unlocked you could insert the USB stick and run a batch file that swithces out your moded boot loader and boot.bmp really fast. You could even alter the autorun file for the USB stick that would run the batch file as soon as it loaded. At least i think that would work....
Sorry its late and I'm knackered.