Changing UI language in Windows

I have an english version of Windows XP pro on which I have installed
a norwegian language pack.

I cannot change the UI language to norwegian however. This is because
the combobox in the "Control Panel"->"Regional and Language Options"-
>Languages->"Language used in menus and dialogs" is not visible.

I have to install another arbitrary language so I have 2 installed
language packs, to get the combobox visible. I can then from the
combobox select norwegian as "Language used in menus and dialogs".

Question #1:
Do I really have to install >1 language packs in order to change UI
language in Windows?

Question #2:
How do I change back to english as Windows UI language? English is not
selectable in the combobox at "Language used in menus and dialogs".
But it is the initial language of my XP installation. I would like to
be able to easily switch between english and norwegian in some way.

thanks

/Marcus