Reformatting form results

Hi, i have a tricky question that i am going crazy about. I dont even have any code to show because i dont know where to start. This is my problem: On my site, i have a popup window that shows a small form, when we fill and send the form, it returns an html page, from another site, with some stuff. What i want to know is if its possible to grab the form results before it reaches the user browser, reformat/change it and then show it. If it is, how can i do it? I dont know where to go about this. Or what to look for. Thanks. Pag