What you download as "avast! BART CD" is not an ISO image. It's an installation program (EXE file) that installs the BART CD Manager. The BART CD Manager is a simple program that lets you
(i) download the latest version (incremental updates)
(ii) generate the ISO.
You then burn the ISO in your favourite software.
Of course, it is recommended to always have as new version as possible on the CD. Hence my recommendation about CD-RW.
Alternatively, you can supply the virus definition file to the virus scanner from an external storage (floppy disk, USB flash disk etc). The virus definition file is identical to that of avast 4 (i.e. it's the file 400.vps from regular avast).
Hope this helps,
Vlk