Re: "You can only install one device at a time."
1. Log on in safe mode, whilst your computer is booting up press F8 repeatedly, you should get a listing of boot options, select Safe Mode.
2. Once you are in safe mode and logged on as the administrator, right click on My Computer, then select properties.
3. Once in properties, select the Hardware tab.
4. Click on the Device Manager button.
5. Click on the + sign by your display adapters.
6. Right click on the onboard graphics controller, and select disable, not install. This will only cause a world of misery and woe. (You might want to click Remove instead.)
7. Log off and turn your computer off.
8. Crack her open, and install your new graphics card.
9. Plug everything back up, and close up the case, turn your computer back on.
10. Once you log in it should say "I found new hardware, gimme a disk,".