I'm setting up a second monitor on my work pc. How do I install a video card without disabling the vga port?

I have a new Dell Vostro running Windows 7. When I connect the ATI video card into the PCI slot, only the monitor connected to the new video card works, the main VGA port is disabled. How can I enable the existing vga port to allow for dual monitors? I had it working on my last pc and now it is not.|||You can't.The BIOS will enable one or the other,your choice,but not both.

Your solution is to get a video card with dual monitor support.It will have 2 VGA or HDMI connections.|||Dual Monitor Graphics Cards are really cheap now a days. Normally motherboard disables online graphics board whenever you connect extranal graphics card.