C code to open, configure and sit listening to a serial port under Windows. Simply edit the port and its parameters (defaults to COM6, 9600 baud, 8 data bits, no parity checking, no flow control and a single stop bit). NOTE if your COM port is already set up you can remove lines 21-42.

serial.c

#include <windows.h>
#include <stdio.h>
#include <conio.h>

int main(void) {
    char port[] = "COM6", buffer[100];
    int i;
    DCB dcb;
    HANDLE hCOM;
    DWORD bytesRead;

    hCOM = CreateFile(port,
        GENERIC_READ | GENERIC_WRITE,
        0,             // REQUIRED
        NULL,          // Default security
        OPEN_EXISTING, // REQUIRED
        0,             // Non-overlapped I/O
        NULL           // REQUIRED
        );

    if (hCOM == INVALID_HANDLE_VALUE) {
        printf ("CreateFile error, code: %d.\n", GetLastError());
        return -1;
    }

    if (!GetCommState(hCOM, &dcb)) {
        printf ("GetCommState error, code: %d.\n", GetLastError());
        return -2;
    }

    /* Port parameters */
    dcb.BaudRate = CBR_9600;
    dcb.ByteSize = DATABITS_8;  // Number of data bits
    dcb.Parity   = NOPARITY;
    dcb.StopBits = ONESTOPBIT;
    dcb.fRtsControl = 0;        // Flow control

    if (!SetCommState(hCOM, &dcb)) {
        printf("SetCommState error, code: %d.\n", GetLastError());
        return -3;
    }

    printf("Serial Port %s configured.\n", port);
    printf("Reading will begin in a moment (press any key to finish)..\n");

    sleep(3000);

    /* Reading loop */
    while(1) {
        if (ReadFile(hCOM, &buffer, sizeof(buffer), &bytesRead, NULL)) {
            for(i = 0; i < bytesRead; i++) {
                printf("%c", buffer[i]);
            }
        } else {
            printf("Error reading from serial port, code: %d.\n", GetLastError());
            CloseHandle(hCOM);
            return -4;
        }

        if (kbhit()) { // Exit on any keypress
            break;
        }
    }

    CloseHandle(hCOM); // Clean up

    return 0;
}