Hi,
This error some times comes due to .NET framework currupted and other can be due to data in cache...
SO i suggest first clear the cache from windows32 folder and get into C:\WINDOWS\Microsoft.NET\Framework
then choose the framework accordingly...
clear the waste contents..Then check all the controls are properly referenced.
Like if ur using some HTML control...mention the full system file ....for example System.IO.* some thing according to the requirements.
and if any variable or control calling from another form dont forget to NEW it....