Finally i got the solution from a forum. I m gonna make it clear
1. Close vs2008 team system if it is open.
2. Go to: start menu>all programs>Microsoft Visual Studio 2008> Visual Studio Tools> Visual Studio 2008 Command Prompt.
3. Write on the command prompt: devenv/resetskippkgs
4. Then open a wpf solution & right click a .xaml file and select open with, u will appear a dialog. U should set windows presentation foundation designer as default designer.