MS-DOS is a relic already right now, and Microsoft is now letting you download the source code for free, from the Computer History Museum.

The source code for MS DOS 1.1 and 2.0 and Microsoft Word for Windows 1.1a is now entirely free and public for the first time. Roy Levin, from Microsoft Research, reminisces:
In 1980, IBM approached Microsoft to work on a project code-named "Chess." What followed was a significant milestone in the history of the personal computer. Microsoft, at the time, provided the BASIC language interpreter for IBM. However, they had other plans and asked Microsoft to create an operating system. Without their own on hand, Microsoft licensed an operating system from Seattle Computer Products which would become the foundation for PC-DOS and MS-DOS...

Following closely on the heels of MS DOS, Microsoft released the first DOS-based version of Microsoft Word in 1983, which was designed to be used with a mouse. However, it was the 1989 release of Word for Windows that became a blockbuster for the company and within four years it was generating over half the revenue of the worldwide word-processing market. Word for Windows was a remarkable engineering and marketing achievement, and we are happy to provide its source code to the museum.