K-lite is a free software that is needed for encoding and decoding various audio and video formats. It is designed as a user-friendly solution for playing all your movie files. Some of the main features of K-lite are

It it always very up-to-date with the newest and/or best components.

• All components have been carefully selected for specific purposes. It is not just a random bunch of stuff thrown together.
• It is very user-friendly and easy to use.
• The installation is fully customizable, meaning that you are able to only install those components that you really want.
• For some formats a choice can be made during installation between different filters. This allows you to tweak the pack to your own specific needs and preferences.
• Uninstallation removes everything that was installed by the pack. Including all registry keys.
• It is extremely easy to make a fully customized unattended installation with the integrated wizard.
• It has been very well tested. The pack doesn’t contain any conflicting or buggy codecs.
• It tries to avoid potential conflicts with other codecs already installed on your computer.
• The pack is able to detect broken codecs and filters on your system, and helps you to remove them.
• It is a very complete package, containing everything you need to play your movies.
• This pack has a huge user base. This means that problems are found and resolved quickly.
• The pack is suitable for both novice and expert users.

And the recent changes that has been made to the new K-Lite Mega Codec Pack 4.0.0 are

• Updated Media Player Classic to version 6.4.9.1 rev. 55
• Updated ffdshow to revision 2035
• Updated DivX to version 6.8.3
• Updated Cyberlink MPEG-2 decoder to version 8.2.0.530
• Updated x264VFW to revision 889
• Updated LAME MP3 ACM codec to version 3.98
• Updated MediaInfo Lite to version 0.7.7.3

It performs great with every video format. If you have this one then you will probably never want to try any other player.

Download : K-Lite Mega Codec Pack 4.0.0

Source : K-Lite