Hi 
I just installed Xubuntu 7.04 onto an old-ish laptop (that was previously running an old release of slackware linux). The initial install wasn't too painful (aside from problems with the disk partitioner returning failure codes numerously, and the fact that the GUI network configurator didn't seem to accomodate ad-hoc WLAN setup which meant me manually poking at the /etc/network/interfaces file)... 

However, I too have just experienced the problem met by various other people in this thread. After the full initial installation, my first port of call was to add a new 'basic' user so that my family could do basic login and have access to web-browsing etc...

so.. i used the GUI tool, went in and selected the add user option, filled out the details of the user then 'OK'd' everything. once i'd done this, i logged out of xfce and then tried to login as the newly created user... i was met with an error trying to log in saying that the account could not be validated.. so i then logged in as my 'initial installation' user account to try see what was going on. After logging in as the initial user.. i tried to run the GUI user accounts tool again.. it prompted me for the administrative password to access the utility.. i put in my usual password and was then met with an error saying that i was not allowed to run this utility... to my horror i subsequently discovered that i could not 'sudo' at all with this account...
what seemed to be particularly drastic is that for some reason I noticed that files in the /etc folder that would normally be root/root (i.e. user/group of root) now seemed to have the ownership of (root/newuser) where newuser was the uid of the account i had just tried to add (which did not get added correctly) with the GUI tool....

as a stop-gap i have booted the live cd and i added my initial login to the sudoers file as an 'ALL=(ALL) ALL' entry.. so i could actually be productive again, and i once again ran the GUI user/group maintenance tool.. upon going in here this time i observed that indeed a new group had been added for the account i tried to set up... trying to remove this made the tool complain that it was an administrator group, therefore i can only imagine that it had relabelled the root group??
i renamed this new group back to 'root' and i tried to add the user once again with the tool and this time it seemed to add the user.

since ubuntu seems 'weird' in its use of the root account and my unfamiliarity with ubuntu, are the files that are retained for administrator access (which are normally root/root on other linux distros) also meant to be root/root in ubuntu? or is the group for these meant to be something else?

p.s. i'm still not sure if this stuff has caused any other havoc at all... but to say such a fundamental tool can screw up so badly is a pretty poor show really.. since i'm not a linux expert i've no idea to what extent it has screwed stuff up and how it may have compromised my system security. 

Its not something i plan to spend major amounts of time on as the install was simply to be put on a laptop to allow my family access to internet etc through WLAN... once that is setup i dont plan on touching it much for a while.. 

i'd like to know how this could be so defective though...