After you have downloaded and installed your codecs, how will you know they work? The best bet in this case is trial and error; you will of course need to test video or music to make sure that the codec works. In the case of Windows Media Player, unfortunately does not come with some of the basic codecs that people are using (hence why you are trying to find, install, and troubleshoot them). Most of the more popular codec packs, like the Windows 7 codec pack, the AC3 pack, and the CCCP will usually solve any issues you've been having.
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In the case that WMP just refuses to play that movie you copied, there are alternative media players, most of them free. A very popular one is the VLC player, which plays a wide range of video and audio. This player is especially good for those who like having digital copies of their DVDs on their computers. From my experience, I have found that VLC is the best player to play such files. Another player is the GOM player. Much like VLC, GOM plays a wide variety of different video and music formats and both of these include their own codecs that you can find in a codec pack.
Both of these players, as well as the codec packs mentioned above can be downloaded for free at Download.com or via their perspective websites.
