There is no need to look for codecs since one pack is all that you need. For example, Combined Community Codecs Pack is the "goto" solution when you need filters to encode or decode files. Access the download page of the application and then save the installation package. Double click to start the installation and configure the things if you know or leave them be as their are.
Restore your PC and Windows Media Player will be able to open many audio and video files.
Note: The pack also contains some video players as well. You can use them to play your files if you find Windows Media Player slow or unresponsive.