Ubuntu 9.10 login screen error

"Ideas? Yes! :)

Solutions? Not sure! :(

It might be a compatibility issue between Compiz (desktop compositing manager using OpenGL and installed by default by Karmic) and your graphic card. If this is the case you could try one of the following:

If you have another spare graphic card, try to run your present installation with the other card and see what happens.
Login from command line (ctrl-alt-F2, for example) and remove compiz by typing sudo apt-get remove compiz, hoping that on reboot, this will trigger ubuntu to automatically switch to metacity (the less fancy version of the desktop manager, default on Gnome). Please note that I did not try this myself, and I am not sure if this will actually totally break your installation or not (although you could try to revert damages by typing sudo apt-get install compiz.
Install an earlier version of Ubuntu, like for example 8.04 LTS. See if that works (I believe metacity was the default back then). Then try to install the proprietary drivers for your graphic card and update the system to Karmic.
Use xubuntu instead! Especially on an older machine, xubuntu feels much snappier than ubuntu or kubuntu... and does not use compiz!

Just my two cents... best luck!"

"is it a fresh install? sometimes with over installs or upgrades, the new install gets confused with the old config files it files.

Failing that, try hitting ALt-Shift-F2 and use the text based log in and see if there is any errors."