By default, Windows XP installs English (United States) as the input language. It is possible to add other languages, for example English (United Kingdom), but if you try to remove English (United States) you are prompted that it is in use and will be removed after the next reboot. This may only be a minor inconvenience, but can be circumvented using an unattended setup file, either for a fully- or partially-unattended build.

Within the [Unattended] section of the answer file, a KeyboardLayout= entry may be specified. The Microsoft Windows Preinstallation Reference states that this entry is used to specify the type of keyboard layout to install during text-mode setup and I have found that by using a United Kingdom keyboard for text-mode setup, no United States entries are created during GUI-mode setup.

The KeyboardLayout= entry must match one of the strings (in quotation marks after the =) in the [Keyboard Layout] section of txtsetup.sif (which is found in the Windows XP installation source).

For reference, my RIS-based unattended installation uses the following entries to specify UK-only regional settings and location:

[Unattended]
KeyboardLayout="United Kingdom"

[RegionalSettings]
Language=00000809
SystemLocale=00000809
UserLocale=00000809
InputLocale=0809:00000809
UserLocale_DefaultUser=00000809
InputLocale_DefaultUser=00000809

Further information on the available regional settings may be found in Microsoft knowledge base article 289125 and the Microsoft global development website has a full list of the available National Language Support (NLS) code pages.