After The client successfully installed , connect it with your github account.It should be easy , just follow the wizard.
Then you should add git.exe location to your "Path Variable". The location you should add will probably be something like :
C:\Users\Your_Username\AppData\Local\GitHub\PortableGit_ca477551eeb4aea0e4ae9fcd3358bd96720bb5c8\bin
Alternatively , if you don't want to add to environment variables. You can open the android studio and go to :
Settings -> Version Control -> Git
In text box next to "Path to Git Executable" you will see "git.exe" , just give it a full path like so :
C:\Users\Your_Username\AppData\Local\GitHub\PortableGit_ca477551eeb4aea0e4ae9fcd3358bd96720bb5c8\bin\git.exe
EDIT :
For latest Github for windows versions some can find the git.exe under "...\cmd\git.exe" rather than "...\bin\git.exe".
That automatically adds its path to the system variable during installation if you tell the installer to do so (it asks for that). So you don't have to edit anything manually.
Just close and restart Android Studio if it's open and you're ready to go.
I encountered a similar error with RubyMine 2016.3 recently, wherein any attempts at checkout or export to Github were met with "Cannot run program 'C:\Program Files (x86)\Git\cmd\git.exe': CreateProcess error=2, The system cannot find the file specified"
As an alternative solution for this problem, other than editing the Path system variable, you can try searching through the program files of Android Studio for a git.xml file and editing the myPathToGit option to match the actual location of git.exe on your computer. This is how I fixed this similar issue in RubyMine.
Posting this solution here for the sake of posterity.