A lot of older motherboards , especially those up to the core 2 era share PCI-e lanes between internal graphics and the x16 pci port. As such it is not possible to use the internal graphics adaptor and the external graphics adaptor at once.
However the sole DVI port is likely a DVI-I port and will output vga with a cheap passive DVI to VGA adaptor. You could use this on your older monitor.
Edit: In this case though, thats obviously not the problem. However now that I know what the motherboard is, finding a solution becomes trivial.
From the manual, page 48, you need to go to the peripherals option in your bios and pick the appropriate setting from the options -In this case IGFX
Init Display First
Specifes the frst initiation of the monitor display from the installed PCI graphics card, PCI Express graphics
card, or the onboard graphics.
Auto Lets BIOS automatically configure this setting. (Default)
IGFX Sets the onboard graphics as the first display.
PEG Sets the PCI Express graphics card on the PCIEX16 slot as the first display.
PCI Sets the graphics card on the PCI slot as the first display.