There are a couple of sample programs provided with VC that could help. The Platform SDK sample uses the Platform SDK functions and such. There is another sample, I think it uses MFC, that uses a VB control to do serial IO. There is also a Technical Article in the MSDN about serial IO.
There are many samples in other web sites of doing serial IO.
Serial IO requires much more advanced programming than writeline("AT"). You must use multithreading. You cannot use just the C++ language, such as writeline, and it will require a lot of time, such as months, for you to develop everything yourself using the Platform SDK. Look for existing code to use; either a library, or a VB/COM/ActiveX control or something like that, or at least sample code that someone else has developed that you can use.