My company has developed a long standing product using MFC in Visual C++ as the defacto standard for UI development. Our codebase contains ALOT of legacy/archaic code which
Depending on the application and the willingness of your customers to install .NET (not all of them are), I would definitely move to WinForms or WPF. Interop with C++ code is hugely simplified by refactoring non-UI code into class libraries using C++/CLI (as you've noted in your selection of tags).
The only issue with WPF is that it may be hard to maintain the current look-and-feel. Moving to WinForms can be done while maintaining the current look of your GUI. WPF uses such a different model that to attempt to keep the current layout would probably be futile and would definitely not be in the spirit of WPF. WPF also apparently has poor performance on pre-Vista machines when more than one WPF process is running.
My suggestion is to find out what your clients are using. If most have moved to Vista and your team is prepared to put in a lot of GUI work, I would say skip WinForms and move to WPF. Otherwise, definitely look seriously at WinForms. In either case, a class library in C++/CLI is the answer to your interop concerns.