That's a good question. XP CDs probably boot via El Torito. El Torito discs have a disk image at the beginning of them. If you could extract that disk image and put it on a hard drive, it should boot. But I don't know at that point how it would handle CDROM access. It might then load CDROM-specific drivers and talk to the drive directly. Or it might not and try to do something crazy to the disk with the image. You could give it a shot. I seriously doubt that you can do it just by copying the disk image though (although that's obviously the path of least resistance, so worth a shot). There's probably a tool out there somewhere to extract the El Torito boot image from a CDROM image, but I don't know where it might be.

I recognize that I'm rambling, but there might be some useful information in there somewhere.