How can root login be enabled in Ubuntu 9.10?
If you already have the root user enabled and are trying to login at the gdm login screen, try checking the file /etc/gdm/custom.conf for the AllowRoot property. It needs to be set to true.
You may have to add those lines to the file. Restart gdm and you should be able to login as the root user.
Also, there is a AllowRemoteRoot property if you are trying to login as root to the machine from another machine.
I really doubt that you need to do this. Just use gksudo anyprogram to run it as root.
And, if you really, really want to run a GUI as root, you can just do (in a text terminal - Alt-1 - Alt-6):
> sudo /etc/init.d/gdm stop > su - (if you already have enabled the root login) # startx
I hardly see any reason to play with the gdm config, as what you are asking for should not be a regular situation, as whenever you need it, you can put the extra effort.
login as other in your ubuntu login screen with username "root" and password that you have set for root. once you logged in, go to system/administration/users and groups and delete your username that you have created while installing ubuntu 9.10. type this in your terminal "sudo reboot" then you was done.