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... so as a quick & dirty workaround, create a new registry key "HKLM\SOFTWARE\MiK\MiKTeX\CurrentVersion\MiKTeX" with a string value "Install Root" set to your MiKTeX installation directory (in my case C:\Program Files\MiKTeX 2.6).

At least this allows the miktexthai installer to work, though it's an ugly cheat.