I get this error only when trying to open older solution files. For instance, I've got VS2013 installed and this error message pops up when I double click on a VS2010 solution file.
Getting rid of it is so simple:
Launch VS2013
Open the old solution file by clicking
"File->Open->Project/Solution..." from the menu (or simply by
pressing Shift+Ctrl+O)
Save the solution with the new format by clicking "File->Save Solution As..." and overwrite the old file.