Since upgrading to Adobe Reader X , I have been suffering a similar problem as described in an Adobe support forum post.
I get the annoying pop-up when I do pretty much anything in Reader, such as pressing the PgDn key, or Ctrl-P for print. After dismissing it, reader usually works OK, but not always. Sometimes it will hang, and needs to get killed from Task manager. This only happens on one of my machines. So it's something about my individual system configuration that Reader dislikes.
Unfortunately, no one responded to the report on their support forum. I understand the software is free and Adobe can't provide individual end user support, but I hope this post will give it this bug more visibility and that it eventually will get fixed.