You do know that although SQL server maybe cheap it can be slower and less reliable that a more mature database like Oracle, particularly for high volume systems like EPOS. SQL server can also throw up data inconsistencies (and support calls) requiring nasty workarounds in client software. MySQL can be better and easier to maintain than SQL Server, for some applications. Different problems can require different solutions, for best results.

I suspect (from other examples) that an all Windows estate would not be as quiet as you hope, given that unplanned and maintenace downtime is often a lot more frequent with Windows systems than with properly maintained Unix/Novell systems. Even DOS systems can be more reliable than Windows, again if maintained properly.

Windows maybe popular amongst Analysts, but analysts have been known to be wrong or even biased!