You can download the PortaBoom CD image with any web browser. Preferred way to download is to use wget command, or something equivalent to it. Here's an example of how to use wget
For Unix systems, wget is available as a source or binary package, depending on your platform and OS. For DOS/Windows systems, there are precompiled binaries of wget available on the SunSite mirror in Denmark; ftp://sunsite.dk/projects/wget/windows/
You can verify that the image that you downloaded has no errors with the md5sum utility. Most Unix systems come with this installed. There's a copy of md5sum available for Windows users linked off of the mirrors page. Here's how you use it:
This generates a 32 byte string (example: "ae63cca262c54ae630095bef3699a727"), that you can compare with what's posted next to the link to the image on the Downloads part of the web page. This is also a good way to determine if you have the latest image or not, the image file's MD5 sum is unique to each PortaBoom image that's released.
Once you've verified that the image is correct, you can then use bzip2 to de-compress it. Unix users should have bzip2 available on their systems, Windows can download a Windows executable of bzip2 from the same mirror page listed above. Here's how you use it:
bzip2 -d portaboom.iso.bz2
After you de-compress the image, run md5sum again on the uncompressed .ISO file, and compare the checksum against what's listed on the mirrors page for the uncompressed image to make sure it's still good. Then you can then use your favorite CD burning software to put the image on a CD ROM.