In England, sentences start with a capital letter. Is Australian
grammar different?
On a more serious note, it might be worth checking all of the regional
settings on your computer. One or more of these settings might have
been set to English (United States).
I can't speak for Vista, but for Win XP, open Control Panel / Regional
and Language options to ensure you have the correct location. Repeat
the process under the "Languages tab" (click on the "Details... option".
Repeat the process under the "Advanced" tab.
Again, I can't speak for Office 2007. However, in Office 2000, you can
change the default language setting, once you have found its hiding
place.
Click on Start / All programs / Microsoft Office Tools / Microsoft
Office Language Settings. Select and install the correct English
version.