Ubuntu官方：制作USB启动盘

http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows

How to create a bootable USB 
stick on Windows

To run Ubuntu from a USB stick, the first thing you need to do is insert a USB stick with at least 2GB of free space into your PC.

The easiest way to put Ubuntu onto your stick is to use the USB installer provided at pendrivelinux.com. You’ll need to download and install and follow the instructions.

Download Pen Drive Linux's USB Installer ›

1. 1

   Select 'Ubuntu' from the dropdown list.

   
2. 2

   Click 'Browse' and open the downloaded ISO file.

   
3. 3

   Choose the USB drive and click 'Create'.