It works!
I only uninstalled the Visual Studio May CTP Orcas Release, but not the Linq May Preview and then install .Net Framework 3.0 RC, Visual Studio Orcas CTP for .Net 3.0 and then the VS 2005 Extensions for WWF RC5.
The Visual Studio Orcas CTP for .Net 3.0 does not include Linq, so the Linq May Preview is the last one.
Enjoy!
Daniel.