Standard Win32 communictation functions allow to control RTS and DTR.
a very nice feature is to have RTS toggled if bytes are to be sent, say you can activate the RS-485 tx buffer via RTS.. and its all done by Windows, without an extra DLL. It works for every Comport, even if its IRDA or an USB-to-RS232 dongle.

Study DCB, "Communications Overview" and "Communication Structures" in MSDN Library - its free on the web at Microsoft.

Cheers, Roger