unfortunately when you install the Spanish version of windows you can't just change the language you would have to reinstall the English version.
I see this from time to time where people buy the computer real close to the boarder like in southern Texas or what not and by chance they end up with the wrong language.
I agree with meuchel, before windows xp they didn't make MUI discs where you could install and add additional languages from the installed default. Since Vista Ultimate and 7 Ultimate you can simply download the MUI you want. You will have to locate another install disc in English, the only way to go!