I am writing this from a customer's house. I have spent well over 20 hours diagnosing this system over the past few months (MONTHS!) and here's the straight poop.
The system was not loading into windows after all the windows updates were performed. After testing all the components, which always tested fine, despite the hard drive clicking on boot, the system was a-ok. This evening I tried removing the video drivers I was using (the latest nvidia drivers for his 7900GTX SLI configuration). The system boots, no files get corrupted, everything is ok. It turns out that the Asus reference boards differ enough that installing anything but asus drivers resulted in catastrophic memory allocation errors which eventually led to the system's total failure. Right now I've got the drivers from the disc installed and am currently installing the latest Windows updates. Things to learn from my experience:
1. Test all components first: This is obviously necesssary to make sure bad parts are not the problem.
2. Install drivers off of manufacturer's disc: If you require later updates (if the user is a gamer, for instance) do so after you make sure everything goes as planned.
3. Don't rule out what may be "Obviously not the problem": Something as simple as drivers can manifest themselves as a completely different problem. In this case the drivers were causing what appeared to be a textbook case of either Hard drive failure or memory failure.
I'll update this post when I get home later tonight with how it ends, but so far so good