When you are ready to install the new card, physically install it, when you switch on press whatever key you need to enter the BIOS, f2 or del usually. Navigate through and DISABLE the onboard graphics. SAVE settings and exit.
Computer will reboot and once windows starts install the drivers for the new card, first off from the cd that comes with the card. This will give you a functional computer. If you then want to change drivers, create a restore point and try them.
You could then uninstall the old nvidea software via add & remove programs.