AVG Rescue CD is Free to download and use but the definitions on the ISO are very out of date so you will need to perform an update download every time you scan a machine.

available as an ISO for use on a CD and as a download to create a USB stick.

Avira Rescue System is available free and the ISO is updated daily. If you download and use straight away there is no need to perform an update prior to scanning. if you use the ISO a few days later the update process will be quick.

Bitdefender Rescue CD is available free and can be run from a boot CD or USB.

If your computer no longer starts due to malware corrupting the operating system, or you suspect the security software has been compromised, you can use the F-Secure Rescue CD to securely boot up the computer and check the programs installed. The Rescue CD can also be used for more advanced repair and data recovery operations.

The Rescue CD contains Knoppix (a derivative of Linux), an operating system that runs completely from the CD and allows access to your computer's Windows operating system and hard disks.

You can also download the Rescue CD updates to a USB drive (minimum 256 MB of free space) using a healthy computer with Internet access. You can use this USB drive to fix a computer that cannot connect to the Internet. Instructions on how to do this are included in the Rescue CD User’s Guide.