If you are running XP, you should run the XP Video Decoder Checkup Utility from Microsoft:
http://www.microsoft.com/downl.....laylang=en

Since you are posting in a Vista forum, you probably have Vista. In that case, try this Media Decoder Utility which is NOT from Microsoft.
http://mediacenterexpert.blogs.....ility.html

Run the appropriate one, then report back here on the output.

I tried to set the default decoder, but it says:"unable to set registry entry-object reference not set to an instance of an object"

The FFDSHOW link will take care of all of the codecs listed below it.

Let's make it easy. Use this one. There is only one choice: http://filehippo.com/download_klite_codec_pack/ What you need are MPEG-2 Codecs. With the klite pack you get this and more.

It's on the top right of the page.

I do believe that the MPEG-2 decoder that Vista is looking for is the one that is used by Media Center. Only decoders that are Microsoft approved can be used.

@spyaaaaa, when you run the Vista MediaCenter Decoder utility, whatever is listed is the default MPEG-2 codec. What does it say on yours? Also, when you click on the drop-down box, what are the available choices? Here's a look at mine:

http://img208.imageshack.us/im.....derqq4.png
http://img208.imageshack.us/im.....hoicf5.jpg

Is it possible that I can get that from the internet or somewhere or another computer that have that DVD decoder?~~~because somehow it is not in my computer

I mean the dvd decoder from Microsoft