Xenix was a version of UNIX sold by MicrosoftCorporation, although they later sold all the rights to it to SCO. It was around in the period 1979-1985.
Part of the sales agreement was that Microsoft would not compete against SCO in the Unix market, meaning Microsoft probably aren't allowed to distribute their own version of a unix-like OS, even if they wanted to.
3 pages link to Xenix:
