NVIDIA Code 12 Error

I'm trying to use two monitors but my current video card (nVIDIA GeForce 6800) doesn't allow me to do that. So i went out and bought another video card (nViDIA GeForce 8400) and was told by the guy at best buy that it could run two monitors.

So i put in the 8400 and installed it and then i got the code 12 error. The 6800 is still working. The 8400 is the one with the code 12 error.

Both of the video cards only have a vga port. So will I be able to run both of these video cards to get the two monitors to work? If so how should i fix this?
 
You likely have IRQ conflicts - try disabling LPT1 and/or one of your Com ports.
 
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