Microsoft SAPI has TTS and is free. Not sure about Vista, but the end user might need a hefty download to enable it. If you don't like the horrible voices that come with it, there are third party ones you can licence. I can't remember where, but I saw one company had an Aussy accent and an Indian accent -very funny.

You can make voices yourself, but it isn't easy. You have to play with the setting and see how they sound.

EDIT> Someone made a TTS plugin for Game Studio using SAPI 5.0. Anyone remember?