Displaying m4a in Media Player 11 Library and media center (Vista)

I have a XP Media Center PC with 250GB Hard drive. On this HDD I have about 30gb music in MP3 format and about 40GB in m4a format. The problem am facing is m4a files are not visible in library. Neither in Windows Media player nor in Media Center. The problem actually started since I installed Media Center on this computer.

Is there any way to get them listed in my Libraries? By the way what could be the problem after installing MCP?

You will need a CODEC for those files and the best I can suggest you is http://www.orban.com/plugin/. Secondly you will need to make some changes in iTunes 7 or later. You need to make sure ALL the tracks have the Album Artist tag filled in. You can select multiple tracks and fill them all in at the same time to save some work. Once done with all these things, go to Windows Media Player and set it to not copy or move the files, only to add a reference to them, and to also not replace existing meta tags etc. That’s it.

That was really helpful. I'm able to get WMP to play my m4a files. However, I'm still having the problem that my WMP cannot add the files to the WMP library. I checked and already had the tag plug-in installed. Wondering any other steps I need to follow ? thank you