You should check your manual first but when you boot up you usually just have to press the "del" button to get into the bios. You have to keep pressing while the computer is booting and you will get to a blue screen which is your bios. Some computers, though, its other buttons to push, like F1 and maybe even F2 or F3, thats why you should look in your manual.
Anyway, once in your bios keep looking you will find a place that mentions your onboard video and that is where you can disable onboard video. Disable onboard video and then make sure you exit bios and save your changes, not just exit bios!!
Once you have saved and exited the bios, let it run a bit, then turn it off and put in your video card and you should be fine. When it boots to windows you can install the drivers for your new card.