If you can't use any of the closed-source solutions you found before, I'm going to assume that you are a software developer and want to use the source code for your own project.
In this case, I would highly recommend the id3lib Tagging Library which is licensed under the LGPL. There are various projects listed on their website which use the library, some of which are fully implemented (and open source) MP3 taggers.
Two implementations I found on their website were Mp3 Tag Tools or EasyTAG. Both are open source, and have Windows binaries available. The only problem, however, is the lack of a command line interface (or at least I didn't see it listed on the project pages). Being an open source tool, it wouldn't be difficult to modify and recompile the application to perform whatever tasks you'd like from a command line.
However, if you're interested a strictly command line based tool, and you are comfortable compiling your own binaries, you might be interested in the id3v2 project. While it is officially released for Linux, I had a quick look at the source code, and you should be able to compile it on Windows with no changes.