Serial port access is part of the standard WIN32 API, you don't need any additional libraries and youcan findmost of what you need on MSDN.
Look up the functions
CloseHandle Closes an open object handle.
CreateFile Creates or opens a file, directory, physical disk, volume, console buffer, tape drive, communications resource, mailslot, or named pipe.
ReadFile Reads data from a file, starting at the position that is indicated by a file pointer. This function can operate synchronously and asynchronously.
WriteFile Writes data to a file at a position that a file pointer specifies. This function can operate synchronously and asynchronously.
And optionally
ReadFileEx Reads data from a file asynchronously.
ReadFileScatter Reads data from a file and stores it in an array of buffers.
WriteFileEx Writes data to a file. This function reports the completion status asynchronously by calling a specified completion routine when writing is completed or canceled and when the calling thread is in an alertable wait state.
WriteFileGather Retrieves data from an array of buffers, and then writes the data to a file.
from
MSDN File Management functions, yes I know they are called file management functions and you want to use a serial port but in true MS fashion they are poorly named and in fact access any handle with a data stream type.
Additionally you will want to look up the specific com port functions available as
Communication Functions.
And here is the overview URL
Communications Resources