It’s very very easy to create a bootable Ubuntu live USB.You just need UNetbootin,Ubuntu .iso image and a usb device.
UNetbootin is a tool,allows you to create bootable Live USB drives for a variety of Linux distributions.
1.Install UNetbootin,install it in Windows or Linux.
3.Connect usb device and then open UNetbootin (Go to Applications->System Tools->UNetbootin).
Check Diskimag,select the .iso file by clicking on marked button at right.Select USB drive at bottom.
Click on OK to start,
After a few minutes,you will see this window.
Now you have this live usb created.
Just,reboot from usb and in a graphic interface select default.