I recommend you to post your inquire on the graphics forum, they may have additional information regarding the problem that you are experiencing.
Had the same issue. Did disable the Aero transparency to get far better results. Also XBMC plays with far lower CPU load than VLC (same 1080p 15Mb/s in all cases). Almost no frames lost with XBMC and no transparency.
You must check supported for GMA offloaded video formats with http://bluesky23.yu-nagi.com/en/index.html#DXVAChecker . If utility screen show only MPEG-2 format support, you have video driver installation issue, reinstall it. If more formats shown, you must check actual codec, with which TV channels received coded. Usually offloading enabled only for VC-1 and H.264 formats, MPEG-4 formats like XVID, DIVX or Microsoft MPEG4 offloading supported only by latest hardware generation and only under Linux (cause there are no needed routines in the Microsoft DirectShow API present in current Windows OSes)