@vesuvius: Installing .NET 3.0 "fixed" certain bugs in .NET 2.0. It might have looked like they were both installed and entirely isolated, but that was never the case. If your app was dependent on one of those bugs/behaviours then you'd have seen the same problem you do now (though possibly it'd be harder to diagnose because they looked unrelated).
.NET 4.0 is a standalone version, requiring no prior .NET versions (they have release service packs for .NET)
If I install .NET 4.5 on a machine with .NET 4.0 (A WPF app for example), will this work? I am also looking to install Exchange 2010 on Windows Server 2012 and it plain does not work. This is not just about bug fixes, the problem is far bigger than your care or want to admit, and your usual ploy of discrediting something you disagree with - reminds me of a certain "Humphrey Appleby" , I can read through it like a warm knife through butter. - not only is it tiresome, it is predictable. It's a recurrent theme in how you deal with disagreement.