My current project at work is a legacy application, written using .NET 1.1. The application is at least 5 years old and has had a wide range of developers. It's complex, has many third-party elements and constraints and lots of lots of code. Like all legacy applications, they set out with best of intentions but ended up somewhere else when new requirements started to deviate from the original design. It's safe to say that it's got challenges, it works despite its bugs and all hope is not yet lost.
Oh, and no unit Tests. Which in my world, is a pretty big thing. Hope you like Spaghetti!!
Fortunately, the client has agreed to a .NET 2.0 migration, which is a great starting place. All in all, I see this as a great refactoring exercise to slowly introduce tests and proper design. Along the way, we'll be fixing bugs, improving performance and reducing friction to change. I'll be writing some posts over the next while that talk about the strategies were using to change our landscape. Maybe, you'll find them useful for your project.