These are links to Anti-virus vendors that offer free LiveCD or Rescue CD files that are used to boot from for repair of unbootable and damaged systems, rescue data, scan the system for virus infections. Burn it as an image to a disk to get a bootable CD. All (except Avira) are in the ISO Image file format. Avira uses an EXE that has built-in CD burning capability.
If you are not sure how to burn an image, please read How to write a CD/DVD image or ISO. If you need a FREE utility to burn the ISO image, download and use ImgBurn.