I'm thinking that unless there is a third party program that does so (which there probably is), you'd have to change window's language settings as a whole to get the on screen keyboard to switch to Cyrillic. If it is a physical keyboard, again, you'll probably have to change the language settings.
за Vista знам че имаш избор он няколко вида кирилица, вкл. фонетичната която трябваше да се добави допълнително. Предполагам Win7 ще я поддържа.
since it is English language forum, here it goes in English:
in Vista there is several Cyrillic types installed. Including the phonetic one which was available as add-in. I guess Win7 still has the same support.