In general its a good idea to allow users to choose appropriate settings, but being able to adjust the default user account settings to provide users with an appropriate default locale is often helpful. Also one cannot easily change the system account's settings.
The Regional and Language Options Control Panel (intl.cpl) has an administrative tab that you can then choose "Copy to reserved accounts" from. This copies the settings to the system and default accounts so that new users get those settings. I'd recommend testing on the local account to make sure that these settings are what you want. The label was slightly different in XP, but it does the same thing.
To do this in an automated manner see "Windows Vista Command Line Configuration of International Settings" is on-line on MSDN. The MSDN article http://www.microsoft.com/globaldev/vista/vista_tools/vista_command_line_international_configuration.mspx discusses changing options using an xml configuration file which you create.