Nowadays we have quite number of extension for both pictures and sounds. Being so damn outdated, Windows XP doesn't have any decent package installed to decode those files. Even for a common extension like rmvb, Windows is unable to open and play it. So, codec package comes as a saviour here.
As a codec package, K-Lite is quite nice. It comes free with plenty of useful codecs packaged inside it. It brings also Media Player Classic, that in my opinion, is far useful than Windows Media Player. It's extremely light resource-wise, compared to WMP and other players like Power DVD, for example. The installer itself is quite self-dependent and doesn't need too much intervention during install, which makes it newbie-friendly. And no, you don't need to worry about hidden spyware inside this software. It's been confirmed clean.
If you want to tweak around its settings, the configurations for each codecs is available on start menu. It includes even a program to detect codecs used on a file, be it movie or music. So, we don't need other codec-recognizer tool. Also, the help section is comprehensive enough, should be able to answer most common problems countered during this package usage. The only downside of this software is... its size. You will need considerable amount of time to download it, seeing it has 30 or so megabytes in size.
The author updates this program regularly. For a piece of free software, we need to congratulate his effort in making this better. Last word, this program is nice and must be installed on every computers.