This codec is unsupported in windows media player and can also be affected by hyperthreading chips (not saying it def will be on your machine but you should be aware of it).
Also the default settings do not allow for the download of the codec so you are stuffed either way but fixing it is simple.
2 solutions, which will definitely work here.
In internet explorer go to Tools>internet options>security.
select custom level for the 'internet' setting.
Change 'download unsigned ActiveX' to prompt.
This worked for me and is documented on the MS website as a valid fix.
This allows Media Player 9 to download the codec it needs because it knows it is there but that setting prevents it downloading it.
If that does not work go to Microsft Windows website search for MPEG 4. There is an issue with HyperThreading technology chips that is known. There is also a fix to download.
Hyperthreading is on newer chips (i.e if its a new 3ghz intel it’s probably hyperthreading.)
Hope this helps, it stumped me for a couple of days.
Thanks again for kind comments,
if anyone is able to I could do with a bit of cross linking to promote a site I am working on.
if anyone has a website you can put a link in to it for me that would be great :)
All the best