Need help on XBMC on Asrock ION 3D

Hello

I just got an ASRock ION 3D and installed WIN XP proff and latest version of XBMC (Eden).
Of course I installed all windows updates and service pack and also updated the Nvidia driver to the latest version.

I have all my files on NAS and the network is wired.

The XBMC works fine until I am playing HD movies (1080 p)
Those movies are not running smoothly.
Both video and audio is non-smooth.

Any clue why this is happening ?
Is it because I am running WIN XP as the OS or do I need to activate any accelrator on the video board `?
Or any settings or codex concerning XBMC I need to be aware of ?

Thanks.