The default programme you have to open Microsoft documents might not be the programme you have installed and confirmed on your computer. As an example, a few years ago I purchased MS Office 2007. Subsequent to this I purchased a new pc with Office 2010 pre-installed. Having paid good money for my 2007 version, I installed it on the new pc use it quite happily for most of the time. However, from time to time I have a hiccup because Windows wishes to use Office 2010 to open a file and it says, quite rightly, that I am not allowed to. What I do then is to open my version of office and then open the recalcitrant file from within my (fully paid up) version of Windows Office. Alternatively I right-click on the file and choose: "open with" and select my legitimate version of Office
cheers, falcybe