Check to make sure that your monitor is not hooked to the mother board connector. If it is, you are probably using the on board video on the mother board. If so do the following:
If the video card you removed was a pci type card, put it back. Hook the monitor to it.
Go into the bios setup when the computer starts up and look for an option to disable the onboard video.
If you find this option disable it.
See if that fixes your problem.
I am currently running Mach on a computer that has onboard video, but the bios allowed me to disable it. I am using an el cheapo PCI video card.
My dad's mini mill on the other hand was a nightmare. I disabled everything, took out all the cards, and used a pci video card all to no avail. There was no way to disable the onboard video even if I ran a pci card. So I just had to replace the motherboard.

Mach just won't work with some on board video.
Hopefully your bios has the option to disable it.
You might also look to see if there is a jumper on the MB to disable the on board video.
If that doesn't work we'll go from there.

Monty