I have a large 95% C, 5% C++ Win32 code base that I am trying to grok.
What modern tools are available for generating call-graph diagrams for C or C++ projects?
Have you tried doxygen and codeviz ?
Doxygen is normally used as a documentation tool, but it can generate call graphs for you with the CALL_GRAPH/CALLER_GRAPH options turned on.
Wikipedia lists a bunch of other options that you can try.