CC,
Start Word, then do File\Open\ Normal.dot
Normal.Dot should be in C : \ Documents and Settings\NAME\Application Data\Microsoft\Templates.
Set the page size to A4 and save - ensure that you save it as a .DOT (template). Now any new docs that you create will be A4 by default.
You may also want to ensure that your locale is correctly set to UK and Language is UK English, keyboard is UK etc. Some of these are in Word \ Options, some in Control Panel \ Regional Settings.
While you are at it, in Word Tools\Options\Print, try checking the "Allow A4/Letter paper resizing".
Also as BOAC says, ensure that you have set up the printer correctly in Printers and Faxes.
SD