The "PCI Serial Port" error on the OptiPlex 780 is a nuisance rather than a critical failure. It represents the Intel Management Engine interface being "blind" to the OS. By installing the (often disguised under the Chipset category), you can clear the Device Manager error and restore the remote management capabilities of this robust, legacy business machine.
Right-click the installer and select . Set it to "Windows 7" and check "Run as administrator." Run the installation and restart your PC. Why Is This Driver Important? optiplex 780 pci serial port driver