The iTunes complains about the fact that the either backup is corrupted or is not compatible with the actual iPhone (I've attached a snapshot.. it's a french pc...).
I have a bakcup of my old 3g iOS 4.2.1 base, while my actual 3gs is equipped with the lqtest iOS 6.1.3.
Take into account that I can open the backup with other trials tools like Dr.Fone, etc.
So I guess that the problem comes from the iOS compatibility or similar.
You could try to remove iTunes and its components using either
Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7
or, if you use mac, just install a fresh copy of iTunes from http://www.apple.com/support/itunes/ , no need to remove anything.
Before you do so, you might want to copy the backup to your desktop to make sure you don't lose it. You can find where is is using
I'm not sure whether this is a solution, but it's the last thing I can think of.