Add a very simple code to an existing source

I have a program that downloads files. It first downloads and checks a list file(*.lst) and then it downloads missiing and/or modified files. During the download procedure there are 2 progressbars that show the completion of the whole progress as well as the download progress of the single file being downloaded at that time. What i need is a button to CANCEL that operation. Just stop it. When you press that button the procedure must end and progressbars must return to 0 position.