Which disk are you booting from? It seems that the old one has a borked master boot record, and for some weird reason (be it hibernation perhaps?) it tries to boot from there. Make sure that the BIOS boot setting is set to the new disk.

In general there is a lot that can go wrong in boot time, and the windows bootloader is an arrogant and secretive prick.
1. It installs on the system boot drive (which may not be what you think it is) and then redirect to the drive where windows is on. Then removing the disk without windows (which seems safe) kills the system
2. XP hibernation modifies something very early in the boot sequence, because after hibernating xp, the pc always wakes up again to xp without asking if you want to boot to any other system.
3. It ignores other OS's (are there any other OS's in the world apart from windows?)

A possible solution is to remove the windows drive, boot into ubuntu, reinstall grub, shutdown, connect the windows drive, boot into ubuntu, run update-grub.

Another possible solution is to download EasyBCD in windows and try to remove all boot flags and bootloaders from the windows drive (not ntldr of course - that is chainloaded by grub) and let grub do the work.