Installing from a USB Key
Some computers lack CD drives, especially in the recently popular Netbooks. USB keys are also more flexible, allowing you to save your files and configuration and to update the key to the latest version of Ubuntu.
As with a CD, you need to get Ubuntu onto the USB key first. Fortunately, there is an easy way to do this in Ubuntu. Under the System > Administration menu, find the USB creator tool. Upon starting, you will see a window asking for an ISO file and a few other options.
If there is an Ubuntu CD already in the CD drive, it will be automatically detected and used. If you have downloaded the ISO, click the Other button and select the ISO you want. Any mounted USB keys will be shown in the second window for you to select.
Now, you must choose whether or not to have persistence. This means that you can save your files and configuration to the USB key. Finally, click Make Startup Disk.
Another way to create a bootable USB key is to use a program called UNetbootin, which has versions and instructions available for both Windows and Linux from unetbootin.sourceforge.net.
Once the disk has been created, you must restart your computer and boot off the USB key. This might involve holding down a key at startup to select from a boot menu or might involve changes to your computer's BIOS. Consult your manual for how to do this.
Once you are booted, your USB key will work exactly like a Live CD and allow you to use Ubuntu from the key or to install it on the computer.