I'm having the same problem as Jadeia (07/07/04) but her solution hasn't worked for me.
I have been using Visual Studio Enterprise 2003. It has been fine for ages. I haven't installed anything recently that should affect it.
Now I am unable to add forms to new or existing projects, I just get the 'the operation could not be completed' error.
I have tried everthing I can think of. I uninstalled visual studio, and everything relating to the .NET framework.
Then I reinstalled .NET (1.1) and the service pack.
Then I reinstalled visual studio.
Still the same problem. This is really getting me down as my work has ground to a halt, and I would appreciate any insight that you may have.