Vista Media Center Sub-titles (closed captions)

Hi
I have Vista Ultimate installed as an overgrade to Premium. How do I turn
off sub-titles (or close captions) when watching TV in media center, it only
appears on some channels and only on the freeview digital (British) service,
I have tried all settings including closed captions. It's so frustrating as I
have purchased external tuners to try and resolve it, whose software is
abysmal for programming etc. The sub-titles etc, do not appear when using
third party software, only in Media Center, and with any tuner configured
within it, which has eliminated any on board hardware issues.

Thanks anyone in anticipation, you'll be my hero forever if I can resolve
this.