I get this error from time to time when I uninstall some MS software that isn’t behaving very well (I’ve gotten it from uninstalling Orcas, SQL Server early drops and other products). I wanted to highlight the two big fixes to this (without resorting to the painful re-installation of VS):
First, tell VS to heal itself (this fixes it about 80% of the time):
If that doesn’t work, make sure that the projectaggregator.dll didn’t get unregistered accidently:
regsvr32 "c:\program files\microsoft visual studio 8\common7\IDE\projectaggregator.dll"