Won't Boot - Not Responsive to Keyboard

So I have an Intel Mac Mini circa 2006 that I was trying to wipe with my OS X 10.5 Leopard Family Pack DVD. The intent was to do a fresh installation of Leopard 10.5.6 and then sell it to buy another computer for grad school. Anyhow, I installed OS X 10.5 from the DVD - no problem. I then applied the 10.5.6 update which I have archived as an image on an external HDD. It took a very long time for the update to complete, hanging at about 80% for about 5 minutes or so before completing "successfully". I then selected the restart button from the dialogue box which popped up. It did the usual restart, and then restarted again by itself before the desktop came up. The double restart seems pretty normal in my experience with larger OS updates. Problem is, it just sits with the grey screen and Apple logo, with the small swirling progress thingy spinning. It does this indefinitely, or at least for 8 hours over night. I attempted to insert the Leopard 10.5 OEM DVD to boot from it, figuring the install went south (perhaps I should have done the 10.5.5 Combo update before the 10.5.6 update). Problem is, the keyboard is unresponsive and apparently will not input any of the commands for firewire target disk mode, boot volume manager/selector, single user mode, eject DVD, etc... but nothing seems to get through. I've noticed the keyboard's caps lock light does not blink upon reboot like I recall my old Apple keyboard doing before. This keyboard is one of the "tiny" new aluminum models with no number pad, but the keyboard seems to work just fine on my laptops. The system boots to the grey screen with the swirling thingy every time and does absolutely nothing. I have attempted the reboot now many times unpluging external HDD's, unplugging ethernet cables, trying different USB ports for the keyboad, etc... I have even tried booting with nothing attached at all, including the monitor - still no luck. The system is not starting networking as it never appears on the network either. Does anyone have any idea's as to what went wrong or what to do next? I am considering opening it up and trying a different HDD perhaps, however the current HDD is not making any noise and the system does not feel excessively warm to the touch. Any input would be greatly appreciated. I obviously would not feel right selling this if it is broken so I'm hoping there is something obvious or simple I am missing.
Thanks,
Dave

Sounds like the archived X.5.6 update is your problem particularly if it was downloaded on an iMac or Tower which may have downloaded components other than what the Mini requires.

Your keyboard and mouse plugged directly into the Mini and not via a hub? if so try unplugging, reconnect, reboot and if you can zap the PRAM by holding down Command, Option P and R keys.

OK, I was able to fix it. Turns out - my best guess - is that it was the archived 10.5.6 update. I yanked the HDD, reformatted it on another Mac then reinstalled 10.5. Everything went fine when I just let the updates run and download from Apple.