BigSteve: With all respect, I'm gonna go out on a limb here, right along side Habitual, and just encourage you to rethink this whole approach: 1) Yes, most distros will ask/force you to set up an initial (non-root) user during the installation process... So why would you need to reinvent this particular wheel? 2) There are several more things to consider and accommodate in setting up a new user account than your Zenity script addresses. 3) Getting sudo and root-privs involved for "a user with no Linux skills can use" is really asking for trouble, esp during the installation/post-installation stage -- why would you open such a hole on a brand new, untarnished box?! 4) Setting up new user accounts is typically considered a sys-admin function, not something for users to do themselves... There's too much mischief, innocent/unintentional or otherwise, to invite here!
My advice -- free, so you can certainly ignore it
-- would be to get yourself one of those nice, thick "Everything about Linux" book/bibles (in Ubuntu flavor, etc) and study it thoroughly before continuing. Such a manual would give you necessary -- and (forgive me) obviously missing -- background and context for setting up and managing your Linux boxes, including things like the rm command, and so much more.
Welcome to the Linux community -- and kudos for your intrepid nature!
But, IMHO, it looks like you're headed into a blind alley here. Get some Linux sys-admin context under your belt -- You'll find/see that there are much more logical and better ways to accomplish what you're thinking of here... Best of luck!