It could be Windows Media Player itself thats at fault. Not any codec.
You can try this to uninstall and reinstall WMP12. It may help.
- Open Start, Type: Turn Fea
- Press enter, and look for Media Features
- Uncheck Windows Media Player and press OK.
- When prompted, restart.
- Repeat this again to reinstall it (but check WMP instead of uncheck).
Windows Media Center also is removed during this process, when doing step 5 you will also need to check Windows Media Center (its right above WMP) if you want Windows Media Center.