The only supported way to set the default handler (besides the Wizard and cmd line properties) is via the UI at Edit > Preferences > General > Default PDF Handler. The installer has to be invoked and many registry entries get touched during the switching process.
You should also verify your dot versions of the product support Windows 8: Overview — Acrobat and Adobe Reader Release Notes.
Finally, while you Acrobat 10 should work with Reader 11, try reverting to Reader 10 (that should be your last resort).
Ben