Philips LCD TV HD ready, how to get an xbox360 to work in HD

A few months ago i bought a philips HD ready LCD TV, i love it... but it wont play my xbox 360 in high definition. I also bought teh VGA HD AV cable that is for the xbox360. i hooked it all up looking forward for the results but the screen keeps saying "video mode not supported", the sound comes through fine, just not the picture. can u please help me sort this out.