#include <stdio.h>
#include <windows.h>
...
string use_Xml = "1";
...

LRESULT CALLBACK WinProc(HWND hWnd,UINT message,WPARAM wParam,LPARAM lParam);

int WINAPI WinMain(...) {...}

    LRESULT CALLBACK WinProc(HWND hWnd,UINT msg,WPARAM wParam,LPARAM lParam)
    {

        switch(msg)
        {

        case WM_COMMAND:

        switch(LOWORD(wParam))
        {

            case IDC_MAIN_CHKBX_X:
            {

                UINT checkedX = IsDlgButtonChecked(hWnd, IDC_MAIN_CHKBX_X);

                switch (checkedX)
                {

                case BST_CHECKED:
                    {
                    CheckDlgButton(hWnd, IDC_MAIN_CHKBX_X, BST_UNCHECKED);
                    SetWindowText(hWnd, TEXT("no"));
                    use_Xml = "0";
                    }
                    break;

                case BST_UNCHECKED:
                    {
                    CheckDlgButton(hWnd, IDC_MAIN_CHKBX_X, BST_CHECKED);
                    SetWindowText(hWnd, TEXT("yes"));
                    use_Xml = "1";
                    }
                    break;
                }

            }
            break;

            case IDC_MAIN_BUTTON_S:
            {
                ofstream myfile ("foo.ini");
                myfile << use_Xml << "n";
                myfile.close();
            }
            break;
        }
        break;

        }

        return DefWindowProc(hWnd,msg,wParam,lParam);
    }

if/else