which codec inside mkv file?

I built an HTPC built on a Zotac ion mobo, and i put a 1core conroe chip in it. the 9300 video chip onboard is supposed to accelerate video decoding, but it doesnt seem to be working all the time. im thinking im missing some drivers/hardware accelerated codecs for the video types that slow it down.

the biggest problem comes when playing some mkv files. the CPU pops up to 100%, and the video gets choppy. so, my question is, what codec is being used inside the mkv to encode the video? as i understand it, mkv is just a container format, so it could be anything. how do i find out what its using under the hood?

MediaInfo will show that for you.

What is MediaInfo?

MediaInfo supplies technical and tag information about a video or audio file. It is free software (free of charge and free access to source code: GPL or LGPL licence)

As an answer to the second part of your question, most GPU's will accelerate mpeg-4/Xvid/DivX as well as standard MPEG-2 codecs.

Most modern GPU's(9300 included) also accelerate h.264. However, BluRay can use either h.264(MPEG-4 AVC), MPEG-2, or VC-1. Often, when a BluRay disk gets ripped, the ripper will directly copy the streams and re-wrap them in a container such as matroska, without any other encoding, so your PC is forced to handle the VC-1 decoding.

No guarantees thats what's happening in your situation, but whether you are decoding VC-1 or something else, that's your likely problem.