1. Find live linux cd image (Ubuntu, Fedora, Debian,…) and download it to your computer

2. Create bootable usb key with dd;

# dd if=/path/to/linux-live.iso of=/dev/sdX 

X – usb key drive (use root not partition), example:

# dd if=/home/username/Downloads/Ubuntu-11.04-amd64.iso of=/dev/sdb

3. Boot from usb-key (while booting your computer press F12 and choose usb key to boot from)

It is as simple as that. 🙂