Problem with DialogBox() returning -1

Hi!

I'm having problems where SOME of my users fail to run my program,
because DialogBox() returns -1. I've set up a MessageBox() that shows
the value of GetLastError(), and it returns 0 (The operation completed
successfully.)

Anyone have any suggestions as to what could cause this?

This is the relevant part of my code. (See http://pastebin.ca/1010305
for hilighted version)

BOOL CALLBACK ConfigProc(HWND hwnd, UINT Message, WPARAM wParam,
LPARAM lParam)
{
    switch(Message)
    {
    case WM_INITDIALOG:
        {
            HICON icon = LoadIcon(GetModuleHandle(NULL),
MAKEINTRESOURCE(IDI_MYICON));
            SendMessage(hwnd, WM_SETICON, (WPARAM)false, (LPARAM)icon);
            loadConfig();
            setConfig(hwnd);
            ShowWindow(hwnd, SW_SHOW);
            return TRUE;
        }
    case WM_COMMAND:
        switch (LOWORD(wParam))
        {
        case IDOK:
            getConfig(hwnd);
            if (!saveConfig())
            {
                /* TODO: Fix this */
            }
            EndDialog(hwnd, IDOK);
            break;
        case IDCANCEL:
            EndDialog(hwnd, IDCANCEL);
            break;
        }
        case IDC_SAVE:
            if (HIWORD(wParam) == BN_CLICKED)
            {
                getConfig(hwnd);
                if (!saveConfig())
                {
                    /* TODO: Fix this */
                }
            }
        break;
    default:
        return FALSE;
    }
    return TRUE;
}

bool showConfigDialog(HINSTANCE instance)
{
    int ret = DialogBox(instance, MAKEINTRESOURCE(IDD_CONFIG), NULL,
ConfigProc);
    if (ret == IDOK)
        return true;
    else if (ret == -1)
    {
        char message[128];
        sprintf(message, "Error when calling DialogBox(): %d (%d)",
GetLastError(), ret);
        MessageBox(NULL, message, "Error", MB_OK | MB_ICONERROR);
    }

    return false;
}

int WINAPI WinMain(HINSTANCE instance, HINSTANCE prevInstance, LPSTR
cmdLine, int showCmd)
{
    if (showConfigDialog(instance))
        return loadAndPatch(cmdLine);
    else
        return 0;
}