a) What else could make the PC lock up like that?
That's the process people usually go through ***before*** concluding that the USB driver is faulty.
So...how come it doesn't crash with the other Serial adapter?
Remember: It goes Wifi->Mega->FTDI.
The magical electrical signal that kills the FTDI's USB port would have to come out of the Arduino's serial port (which is clamped to GND/Vcc) without bothering either the Arduino or the Arduino's USB interface. It would then have to go through the FTDI chip's RX/TX pins (which I imagine are also clamped) and do something bad to the FTDI's USB port. All using a 3.3V DC supply.
We can probably discard that possibility.
That leaves radio.
If being too close to a Wifi transmitter can lock up an FTDI chip badly enough to lock up the host PC, the FTDI driver is at fault.
If it's because it creates crazy voltages in the USB wires which are connected to the FTDI chip, it's still a reason to avoid FTDI chips. There's alternatives which don't have this problem.