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):

devenv /setup

If that doesn’t work, make sure that the projectaggregator.dll didn’t get unregistered accidently:

regsvr32 <YOURVS2005DIRECTORY>/Common7/IDE/ProjectAggregator.dll

or usually:

regsvr32 
  "c:\program files\microsoft visual studio 8\common7\IDE\projectaggregator.dll"

Good luck!