I've heard from a few people that they've had trouble installing Visual Studio codename "Orcas" on their machines.  The problem they've all reported was a popup dialog that appeared when the .NET Framework 3.5 was trying to install.  If you see this error, there's a simple fix:  just go into your "<source root>\WCU\dotNetFramework\dotNetFX35\" folder, go into either the x64 or x86 folder (depending on your computer) and run the setup manually from there.  It ends with no fanfare - it just closes, so watch for that.  After it's complete, Orcas will see that 3.5 is already installed and you can run the regular installer with no additional issues.