Created Issue: can't open or create silverlight projects (VS2010 & VS2012) [11585]

I developed a Silverlight Application (SL4), The application is already deployed (been working for last 3 months and still working in IIS). I can't open the solution anymore. VS2010 keeps promting to install the latest version of Silverlight Developers Runtime ?!. Everything is already installed on my work laptop (I developed the application on my work laptop and deployed it) I copied the same solution to my home laptop (which is offline, no internet connection), the solution opens right away in Visual Studio 2010 and eveything works just fine, so there is nothing wrong with the source code(no version conflict , no other smart guessing here).

Is there any Visual Studio or Silverlight update (SP1 update ?), destroying the development environment, that I am not aware of ?

I also took a new PC. Installed Visual Studio 2012 Ultimate, Installed all Silverlight components (SDK, Runtime, etc for SL5) everything that VS promts is installed, I created a new silverlight project, with an empty grid, cannot debug the application here as well, it keeps asking to install "Silverlight Developer Runtime", when I click to install, it says Silverlight Developer Runtime is already installed ! I jump to SL website and tried to download and instaill Silverlight5 ToolKit , I get the message that it is already installed... It is an endless loop ....