I tried installing nvidia drivers after clean install of kubuntu Fiesty. but installer gives error installation cannot continue with x-server running. i tried init 3 just like in fedora but no response, tried alt+ctrl+backspace but x starts again. tell me how to stop x-server and install nvidia drivers.

Some info here
Worked for me on Dapper

I used the first method from the link below for Feisty on Ubuntu. It worked for me. Make sure you have the proper version of the nvidia driver though for your card. Use the link referenced in method #1 to check your installed card against the driver versions and make sure the referenced repositories are enabled. My GeForce 6600GTOC has run flawlessly ever since. The only snag was needing to enable higher resolution in xorg.conf after subsequently purchasing a larger lcd screen monitor, but the nvidia driver wasn't the fault there.
http://doc.gwos.org/index.php/Latest_nvidia_feisty