Don't know if it's what your looking for.

The Serial Driver API Functions

Function	Description
BuildCommDCB	Builds a device control block (DCB) based on a string containing an MS-DOS Mode command line.
BuildCommDCBAndTimeouts	Builds a DCB structure and sets default timeouts.
ClearCommBreak	Takes a serial port out of the break state.
ClearCommError	Retrieves the most recent error information and clears the error state for the comm port.
CommConfigDialog	Brings up a communications port configuration dialog box.
EscapeCommFunction	Provides additional control of a serial port.
GetCommConfig	Retrieves information about the configuration of a communications port.
GetCommMask	Retrieves the event mask for a serial port.
GetCommModemStatus	Retrieves the status of the modem control lines for a communications port.
GetCommProperties	Used to retrieve information about the capabilities of a communications port and its driver.
GetCommState	Retrieves a device control block (DCB) for a serial port specifying the current configuration of the port.
GetCommTimeouts	Retrieves the current timeout settings for a communications port.
PurgeComm	Clears a communications port.
SetCommBreak	Sets a serial port into the break state.
SetCommMask	Specifies which events on a serial port should be detected.
SetCommState	Configures a serial port according to the specified DCB.
SetCommTimeouts	Sets the timeouts for a communications port.
SetupComm	Sets the recommended buffer sizes for a communications port.
TransmitCommChar	Transmits a single character through a serial port, bypassing the transmit buffer.
WaitCommEvent	Waits on a specified comm event.

And for some function, you need this structure.

Type DCB    '  78 Bytes
        DCBlength As Long
        BaudRate As Long
        Bits1 As Long
        wReserved As Integer
        XonLim As Integer
        XoffLim As Integer
        ByteSize As Byte
        Parity As Byte
        StopBits As Byte
        XonChar As Byte
        XoffChar As Byte
        ErrorChar As Byte
        EofChar As Byte
        EvtChar As Byte
End Type

And no, I didn't try thoses function. They come from Dan Appleman Win32 API Guide.

Hope this will help