struct __regHookKeys {
    short alt;
    short key;
    short flag;
    short  altState;
    short altTime;
    int countAlt;
} _regHookKeys, *regHookKeys;
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc, NULL, NULL);
    regHookKeys->alt=1;//тут ошибка
    return a.exec();
}

regHookKeys->alt=1;

regHookKeys = new __regHookKeys;
regHookKeys->alt=1;