Have you got the proper drivers? Have you uninstalled the onboard graphics drivers when installing the video card?
Latest drivers are installed - 188.8.131.5296 (7/9/2010). Onboard graphics drivers are uninstalled - at first they don't show when the NVIDIA card is installed - took NVIDIA card out, uninstalled onboard drivers (they show in System Properties/Hardware/Display Adapters), reinstalled NVIDIA card. Computer is a DELL Dimension E510 Pentium 4, 3 Ghz processor, with 4 GB memory running Windows 7. Since the screen saver pan and zoom works smoothly using a VGA cable I think the computer is fine - just not sure what else would cause the NVIDIA card to process as expected. Please advise.