Installing 10.6.8 results in "unknown" error

I am selling my late 2006 24" iMac. Since it has the overheating graphics card that some of you are familiar with, I want to have the software smc fan accelerator installed. At first I uninstalled everything I could and reinstalled the 10.6.8 combo, which I had saved on a CD, and erased the white space. Then I noticed i still had a lot of odds and ends in the libraries from myself and the previous owner. Rather than to mess with that, I thought I'd erase the hard drive and reinstall Snow Leopard from the original disk and then 10.6.8.

But then 10.6.8 won't install. It verifies and goes through all the steps, and within half a second of the final install step, it gives me an "unknown error." Disk Utility doesn't find anything wrong with the disk or the permissions. Am I missing an update version, or what else could it be? I created a dummy account that the buyer can change later to his or her information, with no internet connection and a fake phone number (555-555-5555) and address. Could that be the problem? I can't have it in an out of the box state because I have to have the fan app on there for it to behave itself. Plus I thought it would be nice for the buyer to have the latest update on it.