The problem i've stated in the topic occurs on Windows 2000, while the
code works perfectly on Windows XP.

Basically, what I'm trying to do is to recursively enumerate shell
folders starting at the root desktop. The currentFolder parameter for
the first call is obtained from SHGetDesktopFolder.

Dictionary<string, IntPtr> GetSubFolders(IShellFolder currentFolder) {
  Dictionary<string, IntPtr> result = new Dictionary<string, IntPtr>();
  try {
    IEnumIDList enumIdList = null;
    currentFolder.EnumObjects(
       0,
       (int)(EnumOptions.Folders),
       ref enumIdList
    );
    if (enumIdList != null) {
      IntPtr itemId;
      uint fetched = 0;
      do {
        enumIdList.Next(1, out itemId, ref fetched);
        if (fetched == 1) {
          try {
            STRRET name;

            //The following line fails on win2000 once
            //enumIdList.Next has been called a few times:
            currentFolder.GetDisplayNameOf(
             itemId, SHGDN_FORADDRESSBAR, out name
            );

            result[name.sString] = itemId;
          } finally {
            //the pidls will be released by caller
         //if (itemId != IntPtr.Zero) Marshal.FreeCoTaskMem(itemId);
          }
        }
      } while (fetched == 1);
    }
  } catch {

  }
  return result;
}

My Interface/Interop declarations (which work on windows xp):

    [ComImport(), Guid("000214F2-0000-0000-C000-000000000046"),
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IEnumIDList {
        void Next(UInt32 celt, out IntPtr itemId , ref UInt32
pceltFetched);
        void Skip(UInt32 celt);
        void Reset();
        void Clone(ref IEnumIDList ppenum);
    }

    [ComImport(), Guid("000214E6-0000-0000-C000-000000000046"),
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IShellFolder {
        void ParseDisplayName(IntPtr hwnd, IntPtr pbc,
[MarshalAs(UnmanagedType.LPWStr)] string pszDisplayName, out uint
pchEaten, out IntPtr ppidl, ref uint pdwAttributes);
        void EnumObjects(int hwnd, int grfFlags, ref IEnumIDList
ppenumIDList);
        void BindToObject(IntPtr pidl, IntPtr pbc, ref Guid riid,
[MarshalAs(UnmanagedType.Interface)] out IShellFolder /* IntPtr*/ ppv);
        void BindToStorage(IntPtr pidl, IntPtr pbc, ref Guid riid, ref
IntPtr ppv);
        void CompareIDs(int lParam, IntPtr pidl1, IntPtr pidl2);

        [PreserveSig]
        long CreateViewObject(IntPtr hwndOwner, ref Guid riid, out
IntPtr ppv);

        [PreserveSig]
        long GetAttributesOf(uint cidl, IntPtr apidl, ref int rgfInOut);
        void GetUIObjectOf(IntPtr hwndOwner, uint cidl, IntPtr apidl,
ref Guid riid, ref uint rgfReserved, ref IntPtr ppv);
        void GetDisplayNameOf(IntPtr pidl, uint uFlags, out STRRET pName);
        void SetNameOf(IntPtr hwnd, IntPtr pidl, IntPtr pszName, long
uFlags, out IntPtr ppidlOut);
    }

        [DllImport("shell32.dll")]
        private static extern Int32
SHGetDesktopFolder([MarshalAs(UnmanagedType.Interface)] out IShellFolder
ppshf);

        [Flags]
        private enum EnumOptions {
            Folders = 0x0020,   // only want folders enumerated
(SFGAO_FOLDER)
            NonFolders = 0x0040,   // include non folders
            IncludeHidden = 0x0080,   // show items normally hidden
            InitOnFirstNext = 0x0100,   // allow EnumObject() to return
before validating enum
            NetPrinterSrch = 0x0200,   // hint that client is looking
for printers
            Shareable = 0x0400,   // hint that client is looking
sharable resources (remote shares)
            Storage = 0x0800    // include all items with accessible
storage and their ancestors
        }