These aren't freeware, but I've come across two software packages that generate flow charts for C programs: DAC by Ristancase and Understand for C++ (it does C as well) by Scitools.com. I've run the latter more with their demo software and found it OK but any generator will make a kind of mess when dealing with multiple paneled GUIs or many different event sources. Still they're better than nothing when you've got no documentation at all on old legacy code.

JMH