Under no circumstances just erase old software, it MUST be uninstalled !
It should all be found Start>Control panel>Programs>Uninstall a program (Win 7 it'll be similar for XP, Vista etc).
Once the software has been uninstalled, restart the computer and sweep though your system to manually delete any existing files and folders.
Or you can use Driversweeper in Safe Mode to do the job for you
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Once all the old stuff has been uninstalled and deleted, then and only then should you install the latest drivers for the new card. Try not to use those that are on the install CD that came with the card, they're usually several generations out-of-date, download the latest version directly from the Nvidia website, sweep it with your AV software, then run the installer.