Apparently you choose to put the “rescue_disk.iso” file that avast! created onto the CD. The iso file is meant to be burnt onto the CD using a third-party CD burning program, i.e. Nero Burning ROM, CDBurner Xp, etc. The iso file itself is not bootable.

Either install a CD burning program or use one that you have already installed to “unpack” the iso file and burn the results onto a blank CD.