Forget about it - it can NOT be done realtime with general hardware.
Even a high end PC (3GHz P4) has barely enought speed to decode HDTV let alone to encode a 19Mbps stream.
There are consumer HDTV cards (OTA, Fusion III is supposed to support QAM in later fix of their Gold III card) that can decode and record to the hard drive real time but none of them supports XVid (or any other popular encoding codec) as far as I know. I am sure that there are editing systems that studios use to edit HDTV material but we are talking about some obscene prices here.