I wanted to run some of my old XNA projects which gave me a cryptic error message XImporters or TextureImporters missing. When I looked in my references these assemblies where there and properly referenced.
Then I came across this blog Assembly references in XNA Game Studio 4.0 beta projects need to be updated after installing the final release by Aaron Stebner.
As per Aaron’s recommendation I removed all references to the Microsoft.Xna.Framework.Content.Pipeline assemblies and added them again. It worked.
The assemblies are in the following folder: C:\Program Files\Microsoft XNA\XNA Game Studio\v4.0\References\Windows\x86\
The offending assemblies in my project were: