If I understand correctly, .NET is a vision. In fact it is the newly stated vision for the company - your data anytime, anywhere, on any device. As such, a .NET enterprise server helps you to get your data anytime, anywhere, on any device. The .NET framework is a set of base classes which help to accomplish the vision. .NET languages use the framework classes to accomplish the vision.
To be more technical a .NET language is one that targets the common language runtime (CLR). Microsoft has submitted both C# (a .NET language) and the CLR to ECMA for standardization. When approved this makes the CLR, not a Microsoft product but a public product. Just as a developer has license-free use of HTML, so also can a developer implement a CLR for any platform license-free - with or without Microsoft's permission.
As for the CLR being optimized for Windows, certainly Microsoft's CLR for Windows will be optimized for Windows. But if they wrote one for another platform it would be optimized for that platform.
#1 WindowsXP does not ship with ".NET". I doesn't have the CLR or any .NET components. Microsoft gets enough heat for shipping products that many say are still beta, but shipping a product that Microsoft calls beta with a release product? Nope. According to the latest rumors I've heard, VisualStudio.NET goes gold around November along with the CLR.
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