If you have already worked around the installation issue and are using XNA Game Studio on a computer that has the .NET Framework 4.5 installed (including Windows 8 where the .NET Framework 4.5 is installed as a part of the OS), you might notice warnings when opening or building content projects.  The warnings look like the following:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1546,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "Microsoft.Xna.Framework.Content.Pipeline.AudioImporters, Version=4.0.0.0, Culture=neutral, PublicKeyToken=842cf8be1de50553, processorArchitecture=MSIL", "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1546,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "Microsoft.Xna.Framework.Content.Pipeline.FBXImporter, Version=4.0.0.0, Culture=neutral, PublicKeyToken=842cf8be1de50553, processorArchitecture=MSIL", "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1546,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "Microsoft.Xna.Framework.Content.Pipeline.TextureImporter, Version=4.0.0.0, Culture=neutral, PublicKeyToken=842cf8be1de50553, processorArchitecture=MSIL", "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1546,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "Microsoft.Xna.Framework.Content.Pipeline.VideoImporters, Version=4.0.0.0, Culture=neutral, PublicKeyToken=842cf8be1de50553, processorArchitecture=MSIL", "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1546,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "Microsoft.Xna.Framework.Content.Pipeline.XImporter, Version=4.0.0.0, Culture=neutral, PublicKeyToken=842cf8be1de50553, processorArchitecture=MSIL", "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.

This type of warning can occur on Windows 8 (which includes the .NET Framework 4.5 as a part of the OS) or on other versions of Windows if you install the .NET Framework 4.5 using the redistributable installer.

If you encounter warnings like this, you can work around them by opening your .contentproj file in Notepad and adding the following property to the same property group that contains the XnaFrameworkVersion property:

<PlatformTarget Condition=" '$(PlatformTarget)' == '' ">x86</PlatformTarget>

<update date="2/6/2013"> Added a note about the .NET Framework 4.5.  This problem can happen if the .NET Framework 4.5 is installed on any OS, not just Windows 8. </update>