Are you using adobe reader?
If yes, check to see if a different window (excel itself???) is covering the
adobe reader window. (Look at the windows taskbar.)
If there is no entry there, then I bet you need an updated version of reader.
Make sure that all the users have the latest version of the software:
http://www.adobe.com/support/downloads/thankyou.jsp?ftpID=2713&fileID=2551
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I remember having a nag screen pop up from reader and it was hidden by other
windows. It didn't show up on the taskbar, but when I minimized all my other
windows, I could see it. It was very irritating that I had to dismiss that nag
screen (that I couldn't even see!) first.