After you uninstall Acrobat, have tried opening LC Designer 8.1 from the start menu then using the "Set Designer 8.1 As Default" button in Tools->Options->Workspace?
Also, does Designer 8.1 run, and does the preview tab on forms have
the same behavior as workspace?
You could also try opening in Reader before hitting preview, found this in the LC Designer 8.1 help in section "About previewing and testing forms":
>>The Preview PDF tab appears only when Acrobat or Adobe Reader is installed. By default, if you have both Acrobat and Adobe Reader installed, LiveCycle Designer ES starts Acrobat automatically to preview the form. To use Adobe Reader to preview the form, you must start Adobe Reader before you click the Preview PDF tab.