c# - Error in loading assembly -

i new silverlight development , struck on following problem.

i trying add silverlight 3.0 toolkit in visual studio 2008 on windows 7 machine .while browsing add required dll throws error - error in loading type assembly.could not load file or assembly system.windows

check following:

1) visual studio running in administrator mode.

2) installation of toolkit not corrupt. repair if necessary.

hope helps!