I just wanted to share a solution to the "windows media player cannot open this file" problem which i was facing for the past 5 hrs. Since it didn't work in wmp, it didn't work in wmc either.
This type of problem is being reported all over the internet but there are very few solutions that actually work. In my case, the Microsoft DTV-DVD Audio Decoder went missing. I used to tweak WMC settings by using the Win7DSfilter or MCDU to change the default codecs used by WMC. Maybe that was the reason why it happened but still i don't feel for it. Anyone who compares a player like MPC HC and WMC wil certainly see the vast difference in quality.
So today i got this error, looked for solutions for hours and finally i found that the codecs used by microsoft are:

These files are located in C:\WINDOWS\SYSTEM32\
The audio decoder was missing. Fortunately, when you search for "msmpeg" in the find tab AFTER you go to system32 folder, you will see that they are also saved in another folder called "sxs" somewhere in the c:\windows folder and you will also get the location of their license files too. What i did was, copied the missing decoder file from the "sxs" folder and pasted it in system32 folder where all your audio and video codecs are located.
Then, i ran command prompt in administrator mode, typed "regsvr32 msmpeg2vdec.dll" (without quotes) and got a message saying the video decoder was successfully registered. i did the same thing with msmpeg2adec.dll and msmpeg2enc.dll. after registering all three, i restarted windows and tried opening a video with wmp. it still gave an error. Then i removed haali media splitter, reinstalled it and associated all media types with wmp. Now i tried it again and Voila! wmp played all formats flawlessly. then i checked wmc and it too played fine. I remember having such problems before but this one was the most annoying and difficult to solve.

So if you guys ever get such problems, make sure that:
1. you check whether the 3 main microsoft codecs are in place, if not, get it by the method i did.
2. you uninstall your existing media splitter and reinstall the latest version. (this was how i solved most wmp problems in the past)
3. you associate all media formats with wmp. in case of haali, it will ask you automatically during installation.

i'm not using wmp to play my videos. i use mpc hc. but from my experience with wmp, i think it would be best to set it as default but use some other player manually.
I hope this solution would work for many people having this annoying problem.