I had to do it myself on an old computer some time back. It was using the onboard graphics instead of my new Nvidia card. I disabled the onboard in Device Manager and it just switched to the Nvidia no problem. Not saying it's your problem, but it may be..

And:

Blwyddyn Newydd Dda i Chi..

Injustice will be avenged.
Cofiwch Dryweryn.