All of them. I don't have any programs that scan serial ports, I don't have firewall software that blocks access to serial ports, I'm not reading data over the USB with another program...every other solution looks trivial. Board is on, proper port selected, and proper board selected. Nothing is plugged in except USB, and nothing is touching the board.
I've gotten things working on my Linux partition, so it's not a major issue. Would just be nice to get things working on Windows as well.