installing english MUI on japanese XP

hope this is in the right forum!

I run a Japanese Windows XP version on my laptop, and the laptop itself was bought in Japan.
I was wondering what the steps are to change the interface to English.

I've seen what option to change, under Languages and Regions, but the drop-down box in the Languages tab, which is supposed to say "Languages used in Menus and Dialogs" is absent.
Anyone can help me?

Do you have the installation Disk of windows installed on your system? If not, you need to buy a new full version of Windows XP in English or whatever language you want to use and do a clean windows installation again.

I have my own XP. Are you saying it has to be reintalled and in that process I could do it in English?