This problem appear to web because I opened some forms with
Expression Web 4.0 and it seems to have modified my project files.
I finally found that to solve it all I have to do is to:
1. Open the .csproj with a text editor
2. Look for the ToolsVersion an change it from 4.0 to 3.5
And that’s all.
If for any reason you have to debug and develop Silverlight VS 2008,
and you encounter a problem that when you start debugger it reports
something as if the version you have is not supported then first do this:
Open regedit and check:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Silverlight\Components\Debugging\Value
A Version value of 4.0.50917.0 means that you have the Silverlight 4 Developer and you need to:
1. erase this entry.
2. Uninstall Silverlight 4 Tool
3. Reinstall Silverlight 3 Tools for Visual Studio 2008.