As far as I know, having installed both Kubuntu and Ubuntu (so I don't think Xubuntu is so different), you really do pick up a username during the setup. Ubuntu's policy is that the root account (loginname root) is locked by default, i.e. the password is set to unknown, so to enable it you'll need the user account you created during the setup, which has sudo rights.
During the setup you give your name, a preferred username, password for that account (twice) and possibly some other information. After the installation you use that username you gave then, to log in -- with your password. That's the only "working" (in the sense: you can log in with it) account on the machine after the installation, others including root have passwords set to something nobody knows.
If you've managed to forget the account name (loginname), I suggest you boot using the "rescue mode" a.k.a. single user mode, which should bring you into a root account by default. Then simply read /etc/group to determine the username you created yourself (you should know it) and set it's password using passwd or create a new account. By the way, after this you know why you want to set your bootloader a password -- if you don't, basically anyone can use the single user mode to do whatever they like with your computer.