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.