How to run ubuntu from USB key?

It is absolutely possible what you want. There are certain steps that you need to follow to install Ubantu in your USB. The first step to start with is to download UNetbootin software and run it. Select "ISO" from the drop down menu besides "Disk Image". Then navigate to the ISO file that is on the Ubantu installation CD. Then see at the bottom and choose "USB Drive" from the drop down menu of drive type and also choose drive letter of the USB visible next to it. Click "OK".

Once the Unetbootin copies all the files as well as install the bootloader, you need to click on "Reboot Now". If you find that it is booting back to the Windows then you need to reboot again. At this time, you need to change the booting sequence, which can be done from BIOS. In the BIOS setup, you need to specify that the computer should boot from the USB drive at first.

The computer is now configured the way you wanted it to. It would boot from the flash drive and make you use Ubantu. Since, Ubantu is not installed in the computer’s hard drive, anybody can use the actual OS installed in it just by rebooting. However, at this time you need to remove the USB drive.