To validate....

You created a new WinForms app, you did NOT add any of your any code in any way (no assembly references). So the only thing in the solution was Microsoft stuff.

If that is correct and ONLY if that is correct....

As a wild possibility you could try turning off your firewall/anti-virus apps. Then try the new WinForms project. Sometimes they can have wild interactions with system resources including files. Shouldn''t matter but worth a try before proceeding. Of course turn it back after the test.

Otherwise if it was me I would presume something had been corrupted on the system or had been uninstalled.

If WinForms requires some additional MS install (besides standard .Net) then I would uninstall it then re-install. Try the new WinForms test.

Finally I would uninstall the VS IDE and re-install. I would also see if .Net versions (1 through 4) allow for a ''fix'' mode in the install/uninstall options.)

In your project code you missing some assembly file. to know which assembly file is missing in your code you expand the reference option in the solution explorer and the missing file is colored with error mark. to add this file just write click on the reference and add the references. you can add the references from computer, form .net or used project as reference for your project.