DirectX is a multi-media driver that you can download from Microsoft.
To check what version you have, and if it is functioning properly, you can run dxdiag from the run box on the start menu.
Codec stands foe enCOder DECoder.
These are software add-ons for your media player to translate the specific format of digital data into audio and video.
If the DVD movies are true format DVD, you need more than just a codec. You need either a hardware, or software DVD player.
I use Power DVD.I bought my copy on line for under $5.
Check the driver CD for your video card, as it may have a DVD player on it.
Also, if you have a DVD drive on your PC, it should have come with DVD player software.
If you are running Windows XP, there are a couple of updates that you may need from Microsoft that affect the playing of DVDs.
Chas