See the question and my original answer on StackOverflow

Here is a sample C# Console app code that gets the PIDL of current Windows explorer windows:

class Program
    static void Main(string[] args)
        var shellWindows = new ShellWindows();
        foreach (IWebBrowser2 win in shellWindows)
            IServiceProvider sp = win as IServiceProvider;
            object sb;
            sp.QueryService(SID_STopLevelBrowser, typeof(IShellBrowser).GUID, out sb);
            IShellBrowser shellBrowser = (IShellBrowser)sb;
            object sv;
            shellBrowser.QueryActiveShellView(out sv);
            Console.WriteLine(win.LocationURL + " " + win.LocationName);
            IFolderView fv = sv as IFolderView;
            if (fv != null)
                // only folder implementation support this (IE windows do not for example)
                object pf;
                fv.GetFolder(typeof(IPersistFolder2).GUID, out pf);
                IPersistFolder2 persistFolder = (IPersistFolder2)pf;

                // get folder class, for example
                // CLSID_ShellFSFolder for standard explorer folders
                Guid clsid;
                persistFolder.GetClassID(out clsid);
                Console.WriteLine(" clsid:" + clsid);

                // get current folder pidl
                IntPtr pidl;
                persistFolder.GetCurFolder(out pidl);

                // TODO: do something with pidl

                Marshal.FreeCoTaskMem(pidl); // free pidl's allocated memory

    internal static readonly Guid SID_STopLevelBrowser = new Guid(0x4C96BE40, 0x915C, 0x11CF, 0x99, 0xD3, 0x00, 0xAA, 0x00, 0x4A, 0xE8, 0x37);

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("6D5140C1-7436-11CE-8034-00AA006009FA")]
    internal interface IServiceProvider
        void QueryService([MarshalAs(UnmanagedType.LPStruct)] Guid guidService, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, [MarshalAs(UnmanagedType.IUnknown)] out object ppvObject);

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("1AC3D9F0-175C-11d1-95BE-00609797EA4F")]
    internal interface IPersistFolder2
        void GetClassID(out Guid pClassID);
        void Initialize(IntPtr pidl);
        void GetCurFolder(out IntPtr pidl);

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("000214E2-0000-0000-C000-000000000046")]
    internal interface IShellBrowser
        void _VtblGap0_12(); // skip 12 members
        void QueryActiveShellView([MarshalAs(UnmanagedType.IUnknown)] out object ppshv);
        // the rest is not defined

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("cde725b0-ccc9-4519-917e-325d72fab4ce")]
    internal interface IFolderView
        void _VtblGap0_2(); // skip 2 members
        void GetFolder([MarshalAs(UnmanagedType.LPStruct)] Guid riid, [MarshalAs(UnmanagedType.IUnknown)] out object ppv);
        // the rest is not defined

Note: Shell interfaces are only partially defined as we don't need the full details. Feel free to complete it if you need other information.