It was an Adobe Reader issue. This helped eliminating the nagging window...
 
Cancel preparing untagged document for reading
http://www.acrobatusers.com/forums/aucbb/viewtopic.php?id=2478
Alan

 
On Tue, Feb 24, 2009 at 10:34 PM, Alan Stone <software.list.1es9s@gmail.com> wrote:
On Tue, Feb 24, 2009 at 2:23 PM, Hans Hagen <pragma@wxs.nl> wrote:
Alan Stone wrote:
It's a portable with Intel Core Duo CPU P8400 2.26 GHz, 4Gb memory.

hm, then rendering shoul dbe ok; maybe there is something special with your documents
 
Wouldn't know what's so special about them: picture(s), text, some interactive elements, Windows ttf fonts. As it didn't occur when openening the pdf's in Windows XP and Linux, it has something to do with Vista.
 
Alan