(Not to sound mean - but have you googled this at all?)
and have you changed the driver line in you xorg.conf/xfree86.org? If not then, as the root user, open /etc/X11/xorg.conf in a text editor. From there find the section devoted to your video card. In this section is a line which specifies a driver, I believe it reads like this:
Should the driver be set to "nv", change it to "nvidia" and restart the X server by pressing Ctrl + Alt + Bkspc. (Warning - this will log out your user. Save all open files you wish to keep.)
If the line already reads "nvidia" then I suggest you follow that walkthrough very carefully - as I am stumped.