When you say "MS Office" do you mean i the Regional and Language Options?
If not, go to this dialog from Control Panel and do the necessary changes.
If these settings have already been done, then the following may help.
I believe that I had this same problem a while back, possibly worst as I
would get French (France) as well as English (US), very confusing.
Changing the settings within Control Panel did not stop the problem.
Eventually the problem was solved by some investigation by the company's IT
department, who worked with the Registry codes for languages and did
something to stop Word fluctuating between English (UK) and English (US).
English (UK) code is 2057 (LCID Dec) please check on:
http://www.microsoft.com/globaldev/reference/lcid-all.mspx
Since then I have not had major problems with all applications.
That is ablout the limit of my knowledge on how to solve this problem, so
hope this helps
DeanH
