Look something like C++ serial communication.
Your question, however, does not concern Arduino because you know everything how to read Serial messages and evaluate them...
It is just a question of C/C++ which I do not know very much of. I found some approach like
HANDLE hFile =
::CreateFile("COM1",GENERIC_READ|GENERIC_WRITE,0,0,OPEN_EXISTING,0,0);
Maybe some C/C++ expert can explain you or better aks in a C/C++ forum (keyword: serial communication).
Here two links I found concerning your issue: