sudo service gdm stop

will stop your xserver as well. If that doesn't work, try
Code:

sudo service lightdm stop

You can enter those commands while normally logged in.

Then you'll need to do as above.

Once the installation is complete, try restarting your xserver with

Code:

sudo service gdm start

or
Code:

sudo service lightdm start

And checking to see if the problem is fixed. If it isn't the manual says you should run

Code:

sudo nvidia-xconfig

and then restart.