The only times I have managed to lock up a PC using the serial port is when an old PC got a very strong discharge through the serial port.

Wrong baudrates or wrong signal levels should just result in failed transfers. As long as the signal levels aren't strong enough to affect other electronics you should not be able to affect the PC.

But there is one possibility - maybe a serial port driver that doesn't handle all interrupt conditions. So if it receives a framing or parity error, it will exit from the ISR without clearing the error. That could make the PC become almost dead-locked from the huge percentage of CPU time spent with continuous serial interrupts. But it would be scary if someone did send out drivers that doesn't at least clear all interrupt causes.