com0com is a free utility to create virtual serial port pairs which can be used to capture the debug output of ReactOS and direct it to a terminal program.
Useful terminal programs that are able to get this output include:
Get com0com and install it. Run the com0com command shell and type
install PortName=COM4 PortName=COM5
and it will create two virtual ports connected together. If you prefer different ports, just change the port names. A simple help is avaiable via "help" command.
Connecting the virtual machine and terminal application
Connect the Virtual Machine to your first virtual port ("COM4") and the terminal application to the second ("COM5").
The properties of the serial port (usually specified in the virtal machine), like the baud rate, are not important. Instead, be sure to select "NO FLOW CONTROL", because otherwise the terminal app won't receive any characters.