Error in loading assembly

I am new to silverlight development and struck on the followin开发者_开发知识库g problem.

I am trying to add silverlight 3.0 toolkit in visual studio 2008 on my windows 7 machine .While browsing to add the required dll it throws an error - error in loading type from assembly.Could not load file or assembly system.windows

Check for the following:

1) Visual Studio is running in Administrator mode.

2) Installation of the toolkit is not corrupt. Repair if necessary.

Hope it helps!