When Installing Ubuntu from a Live CD via the graphical installer ubiquity you may receive the following error :

              The username you entered is invalid. Note that usernames
              must start with a lower-case letter, which can be followed
              by any combination of numbers and more lower-case letters.

Solution

This error can occur due to the migration assist attempting to migrate documents and settings from other installed operating systems.  To prevent this feature use the --no-migration-assistant option.

root@root:~# ubiquity --no-migration-assistant