This CD let you try Koha. Alternatively it can install both Koha and Ubuntu.
It seems there is no “clean” way to change the admin password. You need to manually change the mysql password and the one in koha-conf.xml and they both must be the same. Reference: http://koha.org/documentation/faq/how-to-change-koha-default-user-name-and-password and here (the important bit): http://blog.chadwollenberg.com/2009/01/26/change-kohan-3-admin-password/