I got loads of errors like this in a VS2008 project the other day. It seemed really confusing since these errors related to files I had not even touched.
It turned out that a dependent project had not been built - which had not stopped the solution building - which meant the project references of other projects had nothing to reference and therefore could not find the metadata.
Looking up the output window to the first error showed what actually caused the problem and fixing this, got rid of the cascaded errors.