I have downloaded ubuntu and saved it on my external hard drive. I would like to replace Windows Vista and run ubuntu on it's own. I have tried to boot straight on to the external usb drive but it won't do this. I moved the .exe file of ubuntu on to the desktop and it said that the device didn't have enough space. It installed 4GB and would not run.
Use unetbootin to "burn" the iso image of ubuntu on the usb drive. http://unetbootin.sourceforge.net/
Don't forget to backup your usb drive!