Kevin,

In VS2003 you can use the Object Browser to navigate through Assembly and class hierarchys.

If you have VS2005 Beta 2 then you can go one step further and drag an assembly, class or interface onto a new class diagram and VS will construct a diagram for you! This is a real cool feature of VS 2005 but I am not sure if you are limited to framework 2.0 or if its respects side by side versioning?

If you would like to navigate from a particular class in a caller/callee fashion then you need to get hold of Reflector. This gives you the added benefit of allowing you to outline/disassemble the class or assembly to see how features have been implemented by MS.

http://www.aisto.com/roeder/dotnet/

Regards
Neil