I don't know how texture intensive your scene is or what type of render settings you are using (even though it may not have any bearing), but I would closely inspect how each model is set up in the material room.  Is there an over use of displacement maps in areas where it isn't necessary (i.e., distance objects or props)?  Are there too many lights in your scene?  If you can keep it simple, do so.  And as you mentioned, it could be 1 object or a combination of objects causing the problem and that it is not happening with all files.

And yes, your pz3 file will get substantially larger when you shut off External binary.

Edit:  This is just an after thought, is PP 2010 installed within the Programs File in W7 or outside?  And, if so, do you have proper authorization?