I work on an English version of Windows XP Pro, but my working language is Spanish. My default input language is set to Spanish, but sometimes Windows changes this setting automatically when you switch applications (and thus you get a different keyboard layout).
Why is that?
(I've become a huge fan of ALT + LEFT SHIFT, which lets you cycle through the available languages.)
Best Answer
The input language settings are per-application. An application which calls a Windows API in order to change the input locale, can set a different keyboard layout, if it is enabled for the user.
Sometimes it also happens, if you accidentally hit the ALT and LEFT-SHIFT keys together. If you don't want to have this behavior, you could "uninstall" all input languages/cultures you don't need in the Control Panel:
EDIT: or just check "Turn off advanced text services".