If you go into the control panel, regional and language options - can you just check that:
On the standards and formats tab the settings are, "English United Kingdom" and "United Kingdom"
On the Languages tab, under "text services and other languages" click "details". From the page that then appears check that "English United Kingdom" has been selected. The UK bit in the bottom of the screen will be bold if it is. Then click Ok.
XP, be default, seems to make everything US English when you install
Hovering over the 'EN' in the task bar will tell you which language you have as default. If you press the Left ALT key and press Shift at the same time then you should see it change to one of the other languages you have installed.