Finding proxy settings programmatically

I am trying to find out a user's proxy settings for his default browser
from C++.

I have seen some other programs that offer to use browser proxy settings
(such as RealPlayer), and I would like to do the same thing.

I can find out the default browser from the registry.

For IE 4.0 settings, I can look in the registry.

For some versions of Netscape (I've checked 3.0), I can
also look in the registry. For 4.5, I can parse the
jsprefs file.

For earlier version of IE, and other versions of Netscape,
I don't have info on where to look, and I don't have the
software. For AOL I have no information at all (I don't even
know if the AOL browser is registered as the default browser).

Has anyone else ever done this in a reasonably comprehensive
way, or at least have info on lots of browser versions?
It would obviously take a great deal of effort to get all
the older versions and find out experimentally.

Some code that can do this would be ideal if anyone has any.