check in the preferences (Ctrl + K) of Acrobat if the option "Display PDF in Browser" under the "Internet" settings is activated.
If not, activate it and then restart Designer.
That's what I was afraid of. I have already tried that, but my previews are still popping up in a new Acrobat window rather than inside LiveCycle. I think this may be something I just have to learn to live with...
You may try to turn off the "Display PDF in Browser" option once, restart Designer, then turn it back on again.
Sometime a registry setting got mixed-up by an de-/installation process.
A repair of Acrobat may also help (Located under Acrobat's help menu).