How to accept incoming bluetooth connection on Windows 7 desktop (with a c++ or c# program)

If you want to do low level stuff like establish a connection via bluetooth through a C# app then you will need to access the bluetooth stack via its API. If your device is running the Microsofts Bluetooth Stack then you can look at 32feet.net's managed wrapper for Microsofts Bluetooth API. If you have the Widcomm stack then you will have to locate Widcomm's API for it.To access the microsoft bluetooth stack, you can use the BthUtil.dll.
BthUtil.dll is an unmanaged dll which is part of the Microsoft Bluetooth stack. It's typically utilised for tasks such as turning the bluetooth radio on or off via a function called BthSetMode
(see MSDN for documentation - http://msdn2.microsoft.com/EN-US/library/aa456007.aspx)

You can check the following links:-

http://www.youtube.com/watch?v=Jn05CU3mxzo

Also, check the 32feet.Net
A Brief description of 32feet.Net

32feet.NET is a shared-source project to make personal area networking technologies such as Bluetooth, Infrared (IrDA) and more, easily accessible from .NET code.
Supports desktop, mobile or embedded systems.
32feet.NET is free for commercial or non-commercial use.
If you use the binaries you can just use the library as-is, if you make modifications to the source you need to include the 32feet.NET License.txt document and ensure the file headers are not modified/removed. The project currently consists of the following libraries:- Bluetooth IrDA Object Exchange Bluetooth support requires a device with either the Microsoft, Widcomm, BlueSoleil, or Stonestreet One Bluetopia Bluetooth stack. Requires .NET Compact Framework v3.5 or above and Windows CE.NET 4.2 or above, or .NET Framework v3.5 for desktop Windows XP, Vista, 7 and 8.

Link
http://32feet.codeplex.com/

Feature link for 32feet.Net
http://32feet.codeplex.com/wikipage?title=Feature%20support%20table&referringTitle=Documentation