Hi Alex.

First of all, I guess that you get this error while trying to debug the
project, not while trying to build it, since the error you get is
"unable to start DEBUGGING....". Is this so?
And have you tried to run the project, but not in DEBUG mode? What's
happening?
I am not an expert, but I myself have met similar problem some times in
the past. And I think it would be useful if you give us some more
information. For example have you create a virtual directory? Or if you
have more than one versions of VS and .net framework, have you set your
project to use the right version of .net framework. And one more
possible solution that comes to my mind. Is your project using
Integrated Windows Authentication? (you can check this in the properties
of your virtual folder, the Security tab...).