You'll need to go into the BIOS and disable the on-board video. Save the changes and power off. (hold the power switch down for 5 seconds) Then reconnect the monitor cable to the new vid card. When you power up, just ignore the "found new hardware wizard" by cancelling out and install the drivers from the disc.
If the is compatible you should be good. If not, you will have to power down and then undo the BIOS changes back to the on-board video. Just be sure to swap out the video connection prior to boot.
Also, check to make sure the card is properly seated in its slot.
There is no need to remove the drivers for the SiS adapter. You never know when you may need the on-board video as a backup. Disabling it in the BIOS should solve the problem.
I got a new graphics card (ATI Radeon 9250, 256mb) and I'm trying to install it in my desktop PC (HP, AMD Athalon). So I've tried uninstalling the drivers for the built-in SiS 760 display adapter, but it won't uninstall! I have gone to [control panel>system>hardware>device manager>display adapters] and uninstaled what is in there, but every time i boot back up and check it, it is still there. :(. If I plug the new graphics card into the PCI bus, nothing come out on to the monitor from either VGA output! Arg, help plz! So how do I completely uninstall the SiS on board display adapter?