Ubuntu error "I've detected a panel already running"

Prerequisite: I use GNOME, not KDE.
I want to install Nvidia driver to my old Ubuntu machine. However, it prompts that a X server is running and the installation can not proceed. Obviously, X server must be killed to install the driver. To press Ctrl+Alt + Backspace does NOT do the work because it RESTARTS the X server instead of kills it.
After searching online, I found that following command worked for me:

sudo /etc/init.d/gdm stop

After I installed the driver, I used command startx to start GNOME. At that time the error "I've detected a panel already running ..." came out. I found this command worked:

killall gnome-panel
killall gdm
startx

In other words, the processing gnome-panel is not terminated even if you press Ctrl+Alt+Backspace to kill the X server.