Cannot Debug using Visual Studios on a Silverlight project

Question

When I try to run a Silverlight project in Visual Studios 2008 I get this error all of a sudden:

Unable to start debugging. The Silverlight managed debugging package isn’t installed.

What have I done wrong?

Answer 1

The answer was I had installed SilverLight 4 when banging around on the web and needed to install it again using the developer verison Silverlight_Developer.exe. I suppose this is kind of a well, duh, but maybe it will save someone else the pain.