Custom Installer in .Net showing Form behind installer

[RunInstaller(true)]
public partial class Installer1 : Installer
{
    public Installer1()
    {
        InitializeComponent();
    }

    public override void Install(System.Collections.IDictionary stateSaver)
    {
        base.Install(stateSaver);
    }
    private void Installer1_AfterInstall(object sender, InstallEventArgs e)
    {
        Form1 topmostForm = new Form1();
        topmostForm.BringToFront();
        topmostForm.TopMost = true;            
        topmostForm.ShowDialog();
  } }

I need to display the topmostForm in front of the default Windows Installer UI. The above is sample code inside my CustomAction that I am using to create a Form. Setting the TopMost property or using ShowDialog is not helping. Is there any other solution to make my form the top most and focussed?