You can't avoid it. Think about the error for a moment:
| Quote: |
"Cannot open registry key for install path of
PlayOnline-compatible software.
ID=1000" |
You moved a system disk and mounted it in another system. This system only knows and cares about one registry, it's own. PlayOnline's entries are in the registry of the other system which your current installation knows nothing about and will never know anything about it. It can't open it and it can't do anything with it. As far as your system and PoL know, PoL's and FFXI's registry entries do not exist. The only way to recreate them is to install PoL FFXI.
That anything else works at all is a miracle, if you genuinely wanted system stability you'd reinstall everything you're using from the old system drive.