Simple Assembly Loading

I'm trying to load a simple assembly from the application directory, but
continually receive System.IO.IOException. I've tried many things, but to no
avail. The basic code is below... It seems like its a security issue?

thanks,
Ivan

Assembly a = null;
AssemblyName an = new AssemblyName();
an.Name = "MainWindow";

try
{
a = Assembly.Load(an);
}
catch (System.IO.FileNotFoundException ex)
{
MessageBox.Show("FileNotFoundException");
}
catch (System.IO.IOException ex)
{
MessageBox.Show(ex.Message);
}