c# code flow mapping/visualization utility?

I've been asked to re-write (from the ground up) an existing C# winforms application. Unfortunately, this code has been modified by at least a dozen different developers over the past three or four years none of whom seemingly adhered to any kind of coding standard. To say that this code base is a mess would be putting it politely. Given the magnitude of the code (~24k lines) and the fact that the code is completely new to me, I'd like to find some kind of utility that will help me to more quickly understand how this application works at a high level. Bear in mind that there don't seem to be an abundance of good OOP practices in this code so I need something a little more detailed than class diagrams. I've seen references to generating sequence diagrams which might be more like what I'm looking for but I only have VS2010 Premium and I'm under the impression that this functionality is only provided with the VS Ultimate SKU. I have access to the current version of .NET Reflector and I've seen a couple of people mention that there are plug-ins for that which might be useful but I don't have any specific names.