The C source code of the once commercial Y&Y TeX distribution (for Windows) was donated to the TeX User Group after Y&Y TeX Inc ceased trading. I bought a copy the Y&Y TeX system in the late 1990s and certainly found it to be an excellent distribution. The source C code is free to download from http://code.google.com/p/yytex/. If you are interested to explore the inner workings of TeX and want an easy to build/compile Windows code base then this should be of real interest. Note that the DVI viewer, DVIWindo, makes use of Adobe Type Manager libraries which are not included in the download; in addition, binaries are not provided so you’ll need to compile them.