I'm assuming Windows if you're using WAMP. Composer likely is just using the PHP set in your path: How to access PHP with the Command Line on Windows?
You should be able to change the path to PHP using the same instructions.
Otherwise, composer is just a PHAR file, you can download the PHAR and execute it using any PHP:
C:\full\path\to\php.exe C:\full\path\to\composer.phar install