SCO v. IBM reopened

The fork(2) system call was implemented with copy-on-write semantics in the POSIX subsystem that was part of Windows NT 3.1 (the first release, in July 1993).

Of course, Microsoft dropped the POSIX subsystem like a hot potato once the government procurement requirement for POSIX compliance (FIPS 151-2) went away. It was no great loss, because the POSIX subsystem wasn't actually useful for anything.