I am installing a new graphics card which I understand requires that in uninstall existing graphics drivers. When I look for graphics drivers in control panel/add-remove programs, I don't find any drivers to uninstall. How do uninstall these drivers? Thanks.

Any drivers in Add/Remove would be listed under the card's manufacturer e.g. ATI or nVidia

Otherwise and also:

Boot into Safe Mode: repeatedly tap the F8 key after the BIOS POST or computer logo screen but before Windows loads.

Device Manager Start/Run/ devmgmt.msc > Display adapter

Right click then click Properties > Driver > Uninstall