Run Linux the easy way - Knoppix

Knoppix is a version of the free Linux operating system. It runs entirely from a bootable CD or DVD, and requires no installation on your computer. It auto detects your hardware and is up and running in under five minutes. It can also be run as a virtual machine under Windows or another OS.

Knoppix works great as a backup environment if your Windows OS is ever compromised or disabled. Download a .iso disk image in either CD or DVD size. Then burn to a CD or DVD. Activate the BIOS setup while booting your computer and enable booting to the CD or DVD drive prior to the hard drive. When you boot with the disc in the drive you will boot to Knoppix and a Windows like interface without pressing any keys. When you boot without the disc you boot to Windows as normal.

You can find additional info on Knoppix here.