Regarding the installation:
VirtualBox)
Some image require the checkbox 'LiveImage' inside the 'storage' section of the VM's property sheet, to be enabled, others must have it disabled.
On hard disk)
Write the iso image to a cd- or dvd-rom , using your favorite burning tool, like Nero or
unetbootin to write it on an usb-stick.
Do a backup of your data to an external storage, like external usb-harddisk or several dvd-roms!
Plug in / insert the dvd/cd-rom or usb-stick and boot of them.
Install Ubuntu as adviced on screen.
hth