I had the same error in recent past, I just kept repeating my composer installation until it was resolved by this way:
1) Download your composer installer (.exe) and then put it on the C:/XAMPP.
2) Run your installer by just clicking on next till the very end.
3) Open your command-line (cmd) and just cd to your project directory (C:/XAMPP/htdocs/myproject) and then type the composer and see if you have it installed on your PC.
4) It must work now, let us say you want to install the PHP framework from the project directory as follows:
cmd=>composer require slim/slim "^3.0".
The solution is very simple you just need to use complete composer path instead of the
composer install eg:-
C:\ProgramData\ComposerSetup\bin\composer install" instead of "composer install
"C:\ProgramData\ComposerSetup\bin\" to the environment variables, so that now you can use as