Some malware cannot be removed from your computer easily, once they get past your anti-virus protection and integrate themselves deeply in your system – sometimes making it unbootable. In such a situation, it may be advisable to use Rescue CDs. A Rescue CD will help you recover your system by removing nasty threat that resist removal by regular antivirus software.
These CDs perform a scan and removes computer virus without booting the computer system. They then allow you to launch the operating system directly from the CD so that you don’t need Windows to be running on your PC. It is from this CD-based operating system that you are allowed to access and launch your hard drive and all your files.
Most bootable Rescue CD builds are usually based on Linux Live CD distribution, which load a simple operating system to scan malware threats without writing any system files on your hard drive. Some use DOS or a DOS clone such as FreeDOS while some use a cut down version of Windows called the Windows pre-installation environment. Rescue CD’s are usually available in the .ISO image file format and are required to be burnt to a media.
Every couple years, a "friend" needs one of these and I need up searching for one... cached here for future reference.