What Codec does an Audio/Video file use?

Here are some tools to help determine what video file is used by a specific audio or video file:

This shows up as a new tab in the file's properties:
http://www.mediatab.co.cc/

A standalone application:
http://mediainfo.sourceforge.net/en

Another (older) application:
http://www.headbands.com/gspot/v26x/index.htm

What codecs are currently installed?

For Windows Media Player (WMP):

To install WMP, go to Programs and Features > Turn Windows features on or off > Media Features.

If WMP is in player mode switch to Library mode
Right-click on an empty an empty area of the tool/menu bar
If you don't see the Help menu, then select Organize > Layout > Show menu bar
Select Help->About Windows Media Player
Click Technical Support Information

For the entire system:

http://www.updatexp.com/sherlock-codec-detective.html

The second post here has some good info on media playback:

http://social.msdn.microsoft.com/Forums/en/csharpgeneral/thread/96acd753-c429-49bc-be54-ba2bce623601

Microsoft FAQ's on codecs

https://support.microsoft.com/en-us/help/15070