I don't think you'll find anything on MSDN. It is possible to do this but the mechanism for storing username and password varies by modem.
Here's a blog that describes several utilities to do this:
How to recover lost ADSL password or other ISP password
Edit: the previous draft contained a link to a similar utility on a web site with domain named "hack-tools". I think Richard was objecting to this (see comments below).
The assumption is the username / password is stored on the host PC in a file or the registry. It might not be.
The following exercise should locate the information:
Export your registry to a text file.
Change your PPPoE user name or password through the PPPoE client software.
Re-Export your registry to a different text file.
Compare the two files (eg. use a tool like WinMerge).
Search your disk for any files with a modified time stamp in between the time stamps of the two exported registry files.
If the information was found in a file, use the Windows file I/O or standard C I/O functions.
Opening a File for Reading or Writing
If the information is in the registry, use this API:
Registy Reference
Good luck.