Recently while installing Xubuntu 12.04 on my old PC, I encounter the problem where Xubuntu was not accepting my root/admin password that I set during the installation. I found the solution to my problem on http://askubuntu.com/questions/24006/how-do-i-reset-a-lost-administrative-password. I'm posting this solution here so that I can keep a record of the problem I've faced and working solutions for them. By default the first user's account is an administrative account, so if the UI is prompting you for a password it's probably that person's user password. If the user doesn't remember their password you need to reset it. To do this you need to boot into recovery mode. Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu that looks something like this: I've noticed on some systems that timing when to hit the left Shift key can be tricky, sometimes I miss it and need to try it ...
