IT LOOKS LIKE CR@P!: Yeah...... I use plain Win32 API, not even MFC. Maybe I will try wxWidgets in the future
Shortest silent period detectable?: Depends on your device & driver really. On board COM ports are much faster, USB-Serial converters are not so good at this.
Why custom baudrate up to 3Mbps?: Because FTDI chips can go up to this speed. Some people may find this useful.
The program stops logging after some time...: The display can only support up to 30k characters. Win32 Edit control was a bad choice arrrrgh!!!!