Vista without Windows Imaging Component update also seems to have problems with 256x256 icons, so I simply reduced the resolution to 128x128.

ILSpy/MainWindow.xaml.cs

@@ -68,10 +68,7 @@ namespace ICSharpCode.ILSpy
 			this.sessionSettings = new SessionSettings(spySettings);
 			this.assemblyListManager = new AssemblyListManager(spySettings);
 			
-			if (Environment.OSVersion.Version.Major >= 6)
-				this.Icon = new BitmapImage(new Uri("pack://application:,,,/ILSpy;component/images/ILSpy.ico"));
-			else
-				this.Icon = Images.AssemblyLoading;
+			this.Icon = new BitmapImage(new Uri("pack://application:,,,/ILSpy;component/images/ILSpy.ico"));
 			
 			this.DataContext = sessionSettings;
 			this.Left = sessionSettings.WindowBounds.Left;