If I were you I'd use a small program called Unetbootin, it allows you to install any OS from a pendrive. Go here (http://unetbootin.sourceforge.net/) and download the Linux version or instead you can use the terminal to install it:
sudo add-apt-repository ppa:gezakovacs/ppa
sudo apt-get update
sudo apt-get install unetbootin
Next, you need to plug your pendrive and download Ubuntu from the Ubuntu website then save the file on a folder or on the desktok.
Then you run Unetbootin (it might ask you to run it as sudo, just enter your password), select the second option and browse to your ISO file and press OK. If your pen drive has enough space this should work, I use an 8GB pen drive.
Finally you restart your computer and go to the BIOS and select external media boot as the first one and install Ubuntu.