Not to toot my own horn here, but GMime is a very complete MIME parser written in C which can handle the Content-Length header. It also handles DOS and UNIX line-endings with ease, handles broken header charset encodings, doesn't require you to read the entire message into RAM, supports PGP/MIME, etc. It also has a very comprehensive set of unit tests that I use to prevent any regressions.
People have been building it on Windows for quite a while now (under cygwin and/or mingw32 afaik), but the past week or so I've been spending time making Windows a first-class priority by setting up Visual C++ Project/Solution files and making sure everything builds.
Figured I'd post even though you've already found a MIME parser just so other people who may have a similar question might see another option if the aforementioned solutions don't suit their needs.
