Go to Control Panel\Add Remove Programs and remove any software\Drivers associated with your existing card
Right click My Computer and select properties\hardware\device manager and expand the Display Adapters entry, click on your existing card and select Uninstall (Red Cross Above)
Ignore warning to reboot (if any) and shut down the computer
Open the case and physically remove the existing card (Taking care to ground yourself first), then install the new card in it's place
Close the case and boot the computer. If possible cancel any "Found New Hardware Wizards" and when the PC is up and running, insert the CD that came with your new card and follow any prompts to install drivers and software for the new card.