Windows Vista Ultimate, Outlook 2007, no Google desktop installed

The following issue: When trying to open or save an attachment (word, excel,
powerpoint), you get a dialogue box asking whether you want to save/open the
file. In the last line, there is a checkbox "Ask always before opening this
file type". This checkbox is gray, and cannot be unchecked (view he
http://www.poetzsch-consulting.de/test/Nachfrage.jpg). Strange enough, this
issue ocurrs with all attachments, except pdfs. For pdfs, for instance, the
box can be unchecked.

How the box can be made accessable? Any clue?
Thanks