I don't see where you're setting the baud rate, parity etc. Most likely these parameters need to be set.
I noticed you're using managed C++. Why don't you use the .NET
Serial Port[
^] object instead of the Win32 API? It is so much easier to use.
Anyway, if you must you Win32 API, here's an article that will point you in the right direction:
Creating a Serial communication on Win32[
^]