I encountered this bug in vs2008 winform C# programs after messing around with installing vs6, vs2005, vs2008 and vs2010 installation. the bug caused the winform not able to render itself during the design time and throws the following error message:

The designer could not be shown for this file because none of the classes within it can be designed

After doing much google search and found not working solution, it happens to me that maybe the .Net 3.5 version in my PC is corrupted. I therefore download the .net cleaning utility from
http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx
and after running the tool to clean .Net 3.5 from my computer and reinstalling the vs2008. everything works!