Web platform installer is a small application that allow you to stay update to date with the latest .NET tools,updates and additions that is being added to the framework.Developers may not have enough time to search and read the latest releases and updates...
After i installed the sp1 beta for Visual studio 2008 and .net 3.5 i get that exception , I'm working on windows xp sp3 . I solved that by reseting All the Code Access security policies , and then restarted the IIS.