I had a problem - I installed a new H.264 decoder. And I already had another 2-3 decoders. And the media players didn't used the new one.
Thankfully one of the decoders is FFDShow from CCCP.
(CCCP is very very nice filter pack - pack of nice decoders and stuff for your media playing pleasure)
So CCCP being very nice gives you the power to disable its filters through its settings. (This could not be said for the other filters I have)
And on top of it CCCP guys have a nice tool which inspects your system and you can unregister/register codecs through it.
The tool is called CCCP Insurgent.
So to find out which codec was f*cking with me I played a file through Media Player Classic, observed which filter it was using - Play menu -> filters. Then I fired the CCCP Insurgent and found the filter and unregistered it. And voila. :)
And if you want to register it back - you should notice where is the location of the file before you unregister it.