Control panel>System>Hardware tab>Device Manger button

In this list you should be able to find a exspanable menu for display devices...you will need to disable the onboard video...do not un-install right click and choose disable...

That should work...

Also you will need to go to the Nivida website and download the new drivers for the card...I use ATI stuff so I dont remember what their site looks like. But it should be easy with the instructions from the website.