I've read about people having problems with usb to serial converters because of the 2 different chip sets used the FTDI chipset is suppose to be the one to use & that is what I have on another computerused, it is also set up as a virtual serial port & has never given me any problems.
Set this one up just the same way.

After starting mach3 & getting a couple of error messages I get the successfully installed message & can reset the stop button that flashes when mach3 opens.
Now if I just try to start the spindle there is a click & the estop starts flashing, tried reducing the speed but same thing just keeps happening.
If I load some gcode & start mach3 then the spindle starts, the machine moves to the correct starting position & then starts to cut the job, it then seems to just stop for no reason, the estop will be flashing & I need to reset it again. Doesn't happen on the same line of code everytime but it is always an estop condition, can't see anything on the diagnostic screen but then again I wouldn't know what I was looking for anyway lol.