If you want to know how to set this up, Shawn’s guide is very comprehensive and lists the DLL’s that are currently included:

• Mscorlib.DLL
• System.DLL
• System.Data.DLL
• System.Drawing.DLL
• System.Web.DLL
• System.Web.Extensions.DLL
• System.Windows.Forms.DLL
• System.XML.DLL
• WPF (UIAutomation*.dll, System.Windows.DLL, System.Printing.DLL, System.Speech.DLL, WindowsBase.DLL, WindowsFormsIntegration.DLL, Presentation*.dll, some others)
• Microsoft.VisualBasic.DLL

Shawn Burke’s Blog : Configuring Visual Studio to Debug .NET Framework Source Code

The one that I personally really like is this one – UIAutomation*.dll