The Content Preparation Progress dialog box shows "Please wait while the document is being prepared for reading" in Adobe Reader 8. This is useful if you use a screen reader or screen magnifier (mostly used by visually impaired users). If you like me don't want it, here is how to remove it and speed up Adobe Reader.
Close Adobe Reader and delete or move this file:
C:\Program Files\Adobe\Reader 8.0\Reader\plug_ins\Accessibility.api
When you open a PDF file next time make sure to select the checkbox "Do not show this message again".