Apktool is a small but powerful utility that allows reverse engineering any compiled Android app (.apk), so you can extract, view and modify the content.
Last available version (2.x) requires at least Java version 7 installed.
To install Apktool in Windows you need to download from Install page a script called apktool.bat and Apktool-2 (apktool.jar).
Copy both files in a folder, for example C:apktool.
Open command prompt with administrative rights and go to C:apktool.
Apktool has various options.
Copy any .apk file in C:apktool.
Then type apktool d filename.apk (where filename is the name of our apk).
After few seconds apktool will create a folder with the same name of the apk and inside you can find all the source files of the app (assets, original, res folders, etc…)