Problem
AVCHD files (MTS or M2TS format) are used by some cameras from Sonya and Panasonic, but many digital players don't support it, so you have to download AVCHD codec if you want to play or edit AVCHD files.
Solution
[Soultion 1] Download FFDshow AVCHD (MTS/M2TS) Codec
FFDshow is a free, Open Source collection of codecs, including AVCHD (MTS/M2TS).
See more detailes here: How to Download FFDshow Codec.
[Soultion 2] Download a Video Player which Support AVCHD (MTS/M2TS)
You can download another video codec, VLC (VideoLAN Client) which is also a media player.
[Soultion 3] Using libavcodec
libavcodec (part of FFmpeg project) is a codec library that supports AVCHD.
FFmpeg is a complete, cross-platform solution to record, convert and stream audio and video. It includes libavcodec - the leading audio/video codec library.
Getting FFmpeg/libavcodec on FFmpeg homepage.
While if you want to edit the AVCHD video file on Adobe Premiere Pro, you'll also need to install Haali Media Splitter apart from the AVCHD Codec.
Tips
See also
How to Fix Codec Error
How to Download X264 Codec
How to Install H.264 Codec for Windows Media Player
How to Install CoreAVC Codec for Windows Media Player