If you are running Vista or Windows 7 with the UAC enabled and have “Run this program as an Administrator” checked on the Compatibility tab for the Visual Studio exe (devenv.exe), you will get this behavior when you try to open a sln file directly from Windows Explorer.
One solution is to go to “%ProgramFiles%\Common Files\Microsoft Shared\MSEnv” or (“%ProgramFiles(x86)%\Common Files\Microsoft Shared\MSEnv” for 64-bit systems) and right-click VSLauncher.exe and select Properties. On the Compatibility tab, check “Run this program as an Administrator“. Now when you try to open a sln file directly from Windows Explorer, you will get the elevation prompt and it will open the correct version of Visual Studio for the file.