If its only codecs that are troubling you my experience is that WMP11 works with all those I've used previously - including those for WMP9 and third party codecs so it may be easier just to reinstall those you need.
You may also need to make sure WMP is using the correct codecs. Some AV apps install codecs that are not fully interchangeable so any version of WMP could encounter problems according to what was installed and in what order.
There is mixed advice on how to uninstall WMP11 but I'd go for system restore or a clean install.
Many thanks to all who replied to my plea. I downloaded some codecs but they didn't work (maybe my inexperience didn't help). Iwent into Add/Remove and thid rolled me back to WMP10. Thanks once again for your suggestions.
This thread is now locked and can not be replied to.