If you get “The operation could not be completed” when trying open a css file in Visual Studio 2010, and you’ve just installed MVC3 or MVC4, then instead of a complete reinstall of VS 2010 (as I’ve seen some suggest), try this first:
go to Tools Menu in Visual Studio, pick “Extension Manager”, in “Online Gallery”, do a Search for “Web Standards Update”. You should see Web Standards Update for Microsoft Visual Studio 2010 SP1, click the Download button in Extension Manager, once downloaded, run the file.
Alternately go to the Visual Studio add on downloads site:
Run the file Web Standards Update.msi then pick repair, and if all is well, you should be back editing CSS files again.