Reason for that is -
Earlier version of the silverlight 3SDK which we work with VS2008 has the version number ” 3.0.40624” and the version which comes pre-installed with VS2010 is “3.0.40818.“
So, all you have to do is go to “program and features(vista,windows 7)/add or remove program(xp)” and uninstall the earlier version of “silverlight 3 SDK version number – 3.0.40624“.
For more details refer -
http://blogs.msdn.com/amyd/archive/2009/10/21/visual-studio-2010-and-silverlight-3-sdk.aspx
