How to install Ubuntu OS in a pendrive

Hi,
I have windows 7 Operating System both in my desktop and laptop. How can I install Ubuntu 11.04 in my pen drive, where I can plug it whenever I need to boot using Ubuntu OS. I know how to have two Operating systems in one computer by having the grub loader and switch between them when required (a dual boot system).
But what I need to accomplish is to have a full fledge OS in my pen drive, so that I can use it with any USB enabled computer. Also I should be able to use the new OS in any new computer irrespective of the hardware drivers (i. e. the new Ubuntu OS needs to pick drivers from internet and install when I plug the pen drive into a new system).
Appreciate your help.
Uthpala.
