Stop Xserver to install Nvidia driver.

If you are using NVdia card, you will have this problem. When you need to update the driver, you need to stop the Xserver before you do it. Here is how you can do it.
1.

/etc/init.d/[gkx]dm stop

whether you use gdm stop or kdm stop or xdm stop depends on what you are using. You need to be root in order to do this.

2. Now you can update your Nvidia driver. Usually you do it like the following:

nvidia-installer --update