Looking at the C# project templates in VS2008 and the offerings are WPF User Control Library, WPF Custom Control Library and Windows Forms Control Library. Which of these would you use if you wanted to move a legacy active control written in c++ into the world of C# and .NET?
It sounds like you are trying to do several different things all at once:
If you have a small codebase you are probably as well to start from scratch and port functionality into the new codebase as required.
For a larger codebase you need to realize that this is an expensive task both in effort and defect rate.
An order might be:
Most importantly be able to justify doing all of the above!