>>Twelve years with 100% availability is a whole other animal. I still can't help wondering if your client has a policy in place, where regular maintenance occurs at, say, 00:00 - 04:00 every Thursday, and that is not counted as a lack of availability. Uptime vs. availability can be defined in different ways: http://en.wikipedia.org/wiki/High_availability .
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>I wonder the same thing myself. I would guess they have something like that; it is hard to imagine not.
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>You said a mouthful with "not Internet facing". I would add "not GUI" and "expected to be maintained only by trained professionals and not by anyone else."
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>I have had Novell 4 and 5 servers that ran over 4 years without coming down.

FWIW for some time now the "GUI" (default shell) on Windows is just another process, explorer.exe. If/when it crashes, the underlying OS will automatically restart it. I'm not sure it has much effect, if any, on the stability of the OS overall.

That said, you can install Server 2008 without a GUI ("Core" installation). And rumour has it Windows Server 8 goes even further in that direction, with a philosophy that is arguably *n?x-like: http://www.theregister.co.uk/2012/03/14/windows_8_server/