From your description, it looks like you are trying to code at a low level to mess with COM ports. Windows documentation specifically states that you must write your own proprietary driver, rather than use the comm.drv driver that comes with windows. You have no choice but to write a driver if you're messing with the NT family anyway, because you can't easily access low-level memory calls because the OS blocks it.

A sample of that driver is in Walter Oney's book 'Programming the Windows Driver Model'.