I had encountered this same problem and found help on the MSDN forums, a patch should be released soon i have been told, However the following works.

1) Open a cmd window at C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE

2) Type "regsvr32.exe ProjectAggregator.dll".


the only remaining issue that i have found is the lack of a workflow designer, or assemblies and i have not received any kind of answer if this will be available or not.