1)Disable the onboard graphics from BIOS if you haven't already.
2)If that doesn't work check that your power supply has enough juice to feed the graphics card.
*www.guru3d.com/article/geforce-8600-gt-and-gts-review-and-shootout/5
3)If that doesn't work take out your PCI-E card reinstall windows and then without installing drivers for onboard gfx plug in the PCI-E card and install it.
4)If you want to skip above step then try to clean your system of installed drivers with some utility,all this assuming that your onboard gfx works.
Hope it helps.