When you first installed Ubuntu you were offered the chance to set the Ubuntu hostname, which is what appears at the command-prompt and is also how your computer is identified should you activate services such as file sharing.
Ubuntu loves to use sudo/gksu to dish out superuser powers, but if you want to permanently enable the root account so you can log into it, type the following, which will assign the root user a password and thereby activate it.
Once the DVD-ROM has booted in live distro mode, and you're run the installation program,
you may get error messages or experience other difficulties. This article offers some solutions
to common installation problems.