Download iperf on the two good and two bad desktops.
Run iperf ( server mode ) to iperf ( client mode ) record the speed between both good desktops
Run iperf ( server mode ) to iperf ( client mode ) record the speed between a good desktops and bad desktop.
Repeat test 2 but put fixed IP addresses on the devices and connect them directly together.
I'm guessing it's the nics on the bad desktops, test 3 would prove this.