If you're using Vista with a nVidia graphics card and you want to update the graphics card's driver, the chances are that after upgrading, you may loose the Aero glass effect (the semi-transparent and blurred windows title bars). That is, Vista may all of a sudden stop using the graphics card's hardware acceleration to draw windows and the desktop, causing the windows' backgrounds to become opaque and the "Switch between windows" 3D effect (the fancy-looking windows switching animation when you press Windows+Tab) to become disabled.
If that hasn't happened to you, then congrats! It happened to me on two computers, one desktop and one laptop. Fortunately, the solution is simple. Just go to "Control Panel -> System And Maintainence -> Check your computer's Windows Experience Index base score" and click "update my score". After the new score has been calculated, you'll get back your graphics acceleration... Don't know why, but it works! :-)