Given an instance of your Form, you can call a MessageBox like this:
MessageBox.show(form, "Message", "Title"); (Check the doc for other parameters.)

However if you want to call this from a background thread (e.g.: BackgroundWorker) you have to use Form.Invoke() like this:

form.Invoke((MethodInvoker)delegate
{
   MessageBox.show(form, "Message", "Title");
});

https://stackoverflow.com/questions/41022980/have-a-message-box-show-on-top-of-installer-or-after-application-closes

I got the message box to show on top of all other applications by calling it like so:

                    MessageBox.Show(
                        "Server registered successfully!", 
                        "Gyptech Micrometer OPC Server",
                        MessageBoxButtons.OK,
                        MessageBoxIcon.Information,
                        MessageBoxDefaultButton.Button1,
                        MessageBoxOptions.DefaultDesktopOnly);

The vital part being that last parameter, MessageBoxOptions.DefaultDesktopOnly. Now the box shows over top of the installer : )

MessageBox.Show(new Form() { WindowState = FormWindowState.Maximized, TopMost = true }, "You clicked Cancel button", "Cancel");

You can use Cursor.Current.

// Set cursor as hourglass
Cursor.Current = Cursors.WaitCursor;

// Execute your time-intensive hashing code here...

// Set cursor as default arrow
Cursor.Current = Cursors.Default;

However, if the hashing operation is really lengthy (MSDN defines this as more than 2-7 seconds), you should probably use a visual feedback indicator other than the cursor to notify the user of the progress. For a more in-depth set of guidelines, see this article.

Edit:
As @Am pointed out, you may need to call Application.DoEvents(); after Cursor.Current = Cursors.WaitCursor; to ensure that the hourglass is actually displayed.