All modern computers can boot from USB media, though often not out of the box. Instructions to enable USB booting are below.
Configure your BIOS:
- enable booting from USB media (this is often on by default)
- reorder the boot sequence so that an USB medium appears before the hard drive.
- Install the rEFIt advanced boot loader
- Insert the USB stick, reboot, and while booting hold the Option key (also labeled Alt)
- Select the boot medium labeled rEFIt
- In the rEFIt menu select (using arrow keys) the item labeled Linux