Debug into the .NET Framework. It's here.  Here's how you configure VS 2008 to use it for debugging.

"Specifically, you can now browse and debug the source code for the following .NET Framework libraries:

• .NET Base Class Libraries (including System, System.CodeDom, System.Collections, System.ComponentModel, System.Diagnostics, System.Drawing, System.Globalization, System.IO, System.Net, System.Reflection, System.Runtime, System.Security, System.Text, System.Threading, etc).

• ASP.NET (System.Web, System.Web.Extensions)

• Windows Forms (System.Windows.Forms)

• Windows Presentation Foundation (System.Windows)

• ADO.NET and XML (System.Data and System.Xml) "

This is huge. Good going!