UserHelper.cs

using System;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interop;

namespace Utility
{
    public static class UserHelper
    {
        [DllImport("user32.dll")]
        public static extern IntPtr GetActiveWindow();

        public static bool ShowDialog(Window dialog, Window owner)
        {
            new WindowInteropHelper(dialog)
                {
                    Owner = owner != null
                                ? new WindowInteropHelper(owner).Handle
                                : GetActiveWindow()
                };
            var dialogResult = dialog.ShowDialog();
            return dialogResult.HasValue ? dialogResult.Value : false;
        }
    }
}

Based on http://sarafianalex.wordpress.com/2008/09/12/wpf-showdialog/