FreeDOS, an MS-DOS compatible operating system has already been available for years, and it's licensed under the GPL which means that you're allowed to sell it (you have to include the source code with the binaries if you do, and you can't place any restrictions on the buyer re-distributing it later, which makes it generally impractical for commercial software but still better than the "suggested donation" thing suggested here.)
Jokes aside, though, I'd advise anyone with actual programming knowledge to not look at the code that Microsoft donated to the Computer History Museum. If you do, and you end up writing code for another project that's vaguely similar to what's in the released DOS code then Microsoft can sue the project in question for copyright infringement. In fact, many open-source projects (FreeDOS included) specifically bar anyone who's looked at this kind of restrictive-licenced code from making contributions to their codebase in order to protect themselves from legal threats.