This may be helpful: http://stackoverflow.com/questions/1673083/delphi-doxygen
However, I was unable to get things working properly with Delphi 5, pas2dox, and Doxygen. While my .pas files appeared to process without error, the generated documentation just a file listing. Clicking on a listed file presented nothing more than a "source link". Clicking that link browsed to a copy of the full-on file source.
I had to bail for time.