Just to clear things up here... Xenix was the name of Microsoft's version of Unix. Yes, there was a Microsoft UNIX
Back in 1979, before MS-DOS was invented, Microsoft's main product was a Basic compiler. UNIX was the main OS in use at the time, so naturally, Billy G decided that he wanted to get his filthy paws on it. MS licensed AT&T's Unix, and combined it with BSD Unix to get Xenix, the first Unix for x86 architecture.
As MS-DOS' popularity sky-rocketed along with IBM's PC (1980 onwards), Xenix suffered a major drop in priority. Microsoft sold the rights to it to the Santa Cruz Organisation (SCO), who later renamed it to SCO Unix. For a most of the 80's, SCO Unix was one of the most popular Unices, but since then it has been overtaken by the offerings of hardware vendors, such as IBM's AIX, Sun's Solaris and Hewlett Packard's HPUX.
Recently, SCO have sold off their Unix division, UnixWare. It was bought by Linux vendor Caldera, who have hopes of getting full Unix branding for their Linux distro.
Some informations taken from Modern Operating Systems by Andrew Tanenbaum