I have Mach3 on my office computer, as well as the one in the workshop.
I have nothing attached to the office computer, but I do have Mach3 set up on it, as though it were running a machine, e.g. the ports and pins are allocated correctly, and all the bits I do not have are all correctly ticked as not being present e.g. none of the switches are activated. (I set it up the same as the workshop computer so I can write GCode programs on it)
I assume you have the drivers installed, and the machine told you it was installed correctly.
What you can do is run a driver test - the program is in the folder (or on your Desktop).
What may be happening is that you are running programs that are waiting for inputs from a switch or something else that you have shown active but do not have.
You might try the Smooth Stepper as a better alternative to a PCI card.
The Smooth Stepper simply plugs into a USB port and provides more inputs and outputs than the standard card. It also generates the pulses for the stepper motor/servo motor drivers - and therefore take a major task off your computer - allowing it to run more efficiently.