If you are having issue with word when double clicking a document or launching an attachment , it could be a corrupt reg key.
open registry editor and rename the following value :
HKEY_CURRENT_USER\Software\Microsoft\Office\version number\Word\Data
where version number is your office version ( 2007 = 12 ) .
you can also do this via command line ( as administrator )
reg delete HKCU\Software\Microsoft\Office\12.0\Word\Data /f
this article has some more good info.