WebClient downloader = new WebClient();
downloader.OpenReadCompleted += new OpenReadCompletedEventHandler(onDownloadCompleted);
downloader.OpenReadAsync(new Uri(DLL_PATH, UriKind.Absolute));  

// Once the assembly is downloaded
private void onDownloadCompleted(object o, OpenReadCompletedEventArgs args)
{
    try
    {
        AssemblyPart ap = new AssemblyPart();
        Assembly assembly = ap.Load(args.Result);
        Object control = assembly.CreateInstance(CLASS_NAME);
    }
    catch (Exception e){}
}