Does XP boot on its own if you unplug the Windows 7 HD? It does not have its own System flag so it may only be booting off the Windows 7 partition which is System - and should also be marked Active. Mark Partition Active
If XP booted on its own on the original system then if you include MBR and Track0 when reimaging it to the new HD, it should boot on its own there. If not you can install EasyBCD to Windows 7 to add XP on the Add OS tab.
To hide either OS in a Windows Dual Boot go to Computer>Properties>Advanced Settings>Startup to set default, time out, or hide an OS.