Just a punt in the dark, but try going to System Preferences > Date & Time > Date & Time tab, and set the year to 2011 or earlier. You can change it back later.
I suggest this because there have been issues with the installer certificates on iLife and some grey installer disc 2 (also iLife) expiring.
Resetting the clock may allow the installer to run.
Amazing. Had the exact same problem as OP, and your tip solved it. Thank you so much.
And to Apple - shame on you! Is it too much to ask that I can actually restore the Apps that came with my MBP, even though it is three years old? You pride yourself on ease of use, but this is really user unfriendly. As a user, I only get the "Unknown error" so to the average Joe, it could be anything really.