It's embarrassing to have to ask this but any help would be really appreciated. I appear to have fouled up my login process. I can no longer fully boot to the Gnome desktop environment that I was prevously using. Here's the story.

I have a Dell Dimension 5000 with 3Ghz CPU and 2 Gb RAM. Running Natty Narwhal with the Gnome desktop enabled, until just now.

I just installed the xubuntu desktop, to try it out as the PC was beginning to struggle for CPU capacity. Used sudo apt-get xubuntu-desktop to install.

Install appears to have worked fine. Changed the login preferences so that the Xfce desktop was the preferred desktop at start up. Tried it out. Wasn't sure so thought I'd alternate between Xfce and Gnome for a while. In order to do that most conveniently (I hoped - and here comes what I think is the cause of my problem) I went back into the login preferences window and changed the desktop preferences to something like "user defined session". I presumed that would allow me to choose my preferred desktop during the boot up process. (I know, I know, I should have checked the help first.)

Now when I restart, the boot up process takes me most of the way into the original Gnome desktop but then finishes just before the upper and lower menu / workspace ribbons appear.

I can run a terminal, which recognises me as the user and the PC name (so brings up the usual command line) but when I type a command like "login", to try to force the boot process to continue, I get the following error message: "Cannot possibly work without effective root".

So as you can see, I'm really stuck here. Can anyone suggest a way forward. (Regardless, thanks for reading.)

Chink

That means you are already logged in.

Find out what the command for the 'login preferences window' is and run it from your command line.

Thanks D_C, I see what you mean and a little trawling around shows that the command is "gdmsetup". I've run that and it does indeed bring up the login preferences window, so I've been able to fix my problem (huge relief as I have lots to do on this machine today). Once again, many thanks for your reply. Regards, Chink