1.Install Unetbootin.Unetbootin is an easy-to-use GUI tool that you can easily create bootable live-usb.
You can download from its homepage,or read this post for installing unetbootin on ubuntu.
2.Download Ubuntu 9.10 iso image from www.ubuntu.com.
3.Open up unetbootin from Applications->System Tools->UNetbootin(For Windows,click to run the exe file you downloaded).Select Diskimage and click marked button to select downloaded iso image.
Click on Ok to start create the bootable live-usb.
4.Once the installation complete,click on Restart Now.Select boot from USB in BIOS,and choose default when you get into an graphical interface.
5.Now,you should boot up ubuntu 9.10 just like boot from live-CD.