I've been using the unregistered versions of PowerISO, and MagicISO, but they both have a 300 MB limit on making image files. Is there any good free software that can do this?
ImgBurn can make ISO files for you. Free to download and use. www.imgburn.com Its also a highly recommended burning program.
I suggest you have a look in this thread:
Ashampoo Burning Studio version 6.75 Makes ISO’s and is free. You can download it from Vnunet.com.
imgburn and burnatonce are both good for this