If you have vista ultimate, download the english(ingles) language pack in windows update, once you're done, type in region and language(in spanish, sorry, i don't know the spanish word for it) into the search box in the start menu, then click on change display language(in spanish), select the english language pack, then it'll ask you to log off, do that, then you should be in english mode.
If you don't have vista ultimate, then you can't change it, your only option now is to buy an english version of vista.
