We have upgraded from QGis 1.8 to QGis 2.2. Qgis 2.2 is first installed, then 1.8 removed. Since then it is impossible to open .qgs-projects directly from windows explorer. We have checked explorer, file extensions and relinked to C:\Program Files\QGIS Valmiera\bin\qgis.bat . But it doesn't help.
We get the error 'qgis_core.dll cannot be found, please reinstall'. But that is rather drastic.
I use windows xp.