In a nutshell here are the steps to be executed:
- Assumptions: there is a bootable MBR already present, either though Linux or Windows.
- Get a bootable USB drive and the CD image of the ubuntu distribution.
- Install the CD image on the usb drive using software say: PendriveLinux.Com
- Make sure that software goes fine, else it may crib about some error if the usb drive is not bootable.
- Get a hold of plop boot loader. This guy allows you to boot to USB drive through its boot loader.
- Reboot into plop boot loader and you are good to go!