You may experience this problem when you try to run an existing SilverLight project.

I only know the quick and dirty solution to this problem. (at least, it worked for my app)
You’ll have to open the .xap file with an archive program like winara. Edit the AppManifest.xaml file, and change the version number into the current number (something like 2.0.30825.0). And your program should work.