In the main cforms plugin folder, find the file called my-functions.php. You can add functions to that file as needed. Its pretty well-commented.
What you want to do is add a function called my_cforms_logic or my_cforms_action, which get triggered at several different hooks during form submission and processing.
According to the docs, you can move the my-functions.php file (annd any css tweaks you make) into a separate folder called “cforms-custom” to protect your customizations from future upgrades…