Here is an answer from another board.

For the most part, each copy of Vista comes in one distinct language. (Exceptions, however, do exist, such as in India where there are multiple dominant languages.) If you wish to change the language you must download a Multilingual User Interface Pack (MUI). Unfortunately such is only supported by Vista Enterprise and Vista Ultimate, so as a Home Premium user there is nothing you can really do unless you are willing to either upgrade to Ultimate or purchase an English copy.

Paul