Re: copying HD to new computer
did you happen to check in your bios to make sure that the drive you want to boot from is the chosen drive? Just telling your pc to "boot from hard drive, before another bootable source" might not be enough. For example, my BIOS lists all 4 of my hard drives individually in the boot order list, along with floppy, and cd drives. Perhaps this is why your pc seems to only want to boot from the new sata drive rather than your 30 gb ide drive.
Tommy Boy is correct, that XP probably wont allow you to install your old drive into another pc and start up. I say probably, cause I have at times been successful in doing what you're trying to do, but not always. Think it has to do with the newer versions of XP Pro ... like XP Pro SP1a and whatnot. Might be that the ones I've had success with were pre-SP1 or SP2 versions.