Using Virtual Serial Port SDK you can create a "new" serial port on your computer. This serial port is completely controlled by your application. Other 3rd party applications can connect to this serial port and believe it is a normal physical port. In this way you can emulate all kinds of hardware normally connected to a serial port, emulate a null modem, and so on.
Virtual Serial Port SDK is available for .NET and ActiveX, on all Windows platforms Windows Mobile/Pocket PC, Windows XP/2k/NT and Windows ME/98. Samples in C#, VB.NET and VisualBasic 6.0 are included.