Unless you are using Vista Ultimate or Vista Enterprise you can NOT change the display language. The only option is to buy an English version and install it or upgrade to Ultimate.
This is an extract from a Microsoft support site:
With additional language files, you can change the display language on your computer so that you can view wizards, dialog boxes, menus, Help topics, and other items in Windows in a different language.
There are two types of language files:
Windows Vista Multilingual User Interface Pack (MUI). Windows Vista MUIs provide a translated version of most of the user interface. MUIs require a license to be used and are only available with Windows Vista Ultimate and Windows Vista Enterprise. If you are using Windows Vista Ultimate, you can download MUIs by using Windows Update. If you are using Windows Vista Enterprise, contact your system administrator for information about installing additional languages.