I cannot really recall the last time I installed a codec pack on one of my PCs. Codec packs add a large number of media codecs, audio and video, to the system so that all programs on the system can use them to play multimedia contents.
This is important for programs that do not come with their own set of binary codecs or lack codecs that you need to play media files. This includes Windows Media Player for example, which cannot play many popular formats out of the box.
Critics of codec packs believe that this is overkill, considering that you may only need a single codec to play all media files that you own, while a codec pack may install dozens of them on your system that you never make use of.
One of the most popular codec packs around is the K-Lite Codec Pack. It is available in various editions, Basic, Standard, Full and Mega that all built up on each other. Up until now, you had to select one of the available editions and whether you needed the 32-bit or 64-bit version. The latter is a thing of the past with today's update to K-Lite Codec Pack 10.0.
All editions of the codec pack ship with unified 32-bit and 64-bit installers so that you only need to make up your mind on the edition that you want to install.
K-Lite Codec Pack comparison
Basic ships with support for a variety of media formats including avi, mkv, flv, flac, ogm and more.
Standard includes everything that basic has to offer plus the tools Media Player Classic Home Cinema, MadVR and MediaInfo Lite
Full includes everything that standard offers plus GraphStudioNext and a couple of additional Directshow filters such as ffdshow and Haali Media Splitter.
Mega includes everything that full has to offer plus AC3Filter, several ACM and VFW codes and a few extra tools.
I suggest you select Advanced Mode during installation as it provides you with customization options that the other modes do not provide you with.
Alternatives depend primarily on the program you are using or want to use. If you are flexible in regards to that, I would suggest you switch to either VLC Media Player or SMPlayer as they both ship with their own codec sets so that you can play virtually all media formats right out of the box.
If you want to use a program like Windows Media Player, then your best option is to install codecs individually. Use a program like Video Inspector to find out which codecs are missing and install only those on your system.
The new setup should make it easier for users to pick the right version for their operating system.