New Post: System.Xml.Xpath error on window xp for Visual Studio 2008

I installed htmlagility pack and developed a new program but when I run the program. I have the following error.

The type 'System.Xml.XPath.IXPathNavigableS' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml.XPath, Version=2.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

I searched System.Xml.Xpath dll in C:\Program Files\Microsoft SDKs and there is no Silverlight folder and no dll. I installed Silverlight 5.1. When I searched it in C:\Program Files\Microsoft Silverlight, I only found System.Xml.dll but no xpath.dll, Please give me any suggestions to move on and start using htmlagilitypack.

Thanks.