Windows XP is no longer supported by Microsoft. The current .NET versions are also not supported on Windows XP, though as you mention .NET 4 on SP3 is.There are reasons that Microsoft did not extend support to pre-SP3, so assume that some things just do not work. I hate to quote marketing material, but in this case it is dead on.

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"Installation of the Microsoft .NET Framework 4.5 is not supported on this operating system. Contact your application vendor." ** And then of course, when I try to install .NET 4.5 on the Win XP machine, I find out it's not supported. At this point, I'm just wanting to know what can be done to get this app to run on Win XP.

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.NET Framework 4.5 Developer Preview not on Windows XP Jun 27, 2012