How to detect Windows 64-bit platform with .NET?

In a .NET 2.0 C# application I use the following code to detect the operating system platform:

string os_platform = System.Environment.OSVersion.Platform.ToString();

This returns “Win32NT”. The problem is that it returns “Win32NT” even when running on Windows Vista 64-bit.

Is there any other method to know the correct platform (32 or 64 bit)?

Note that it should also detect 64 bit when run as a 32 bit application on Windows 64 bit.