I finally got this accomplished on both my desktop (NVIDIA GeForce 8300 GS) and my laptop (NVIDIA GeForce 8400M GS). And this was after multiple failures.
For the desktop, I disconnected all peripherals (except monitor, mouse and keyboard). I already knew that I couldn't run any NVIDIA driver update past 306.97 (Oct 2012), or it would freeze and crash the computer and constantly cycle on the BSOD. So, I
tried unplugging the Ethernet cable after the first restart, but the 8.1 installer had already downloaded the updated driver, so I was stuck with a machine that not only wouldn't roll back to Win 8, but that would freeze and have to be hard restarted.
So, I did what I've learned to do when an updated NVIDIA driver gets downloaded from Win 8: I work as fast as possible to remove the updated driver before it freezes and crashes. In the past, I only had to roll back to the previous driver, but after Win
8.1 installed, it removed the driver that actually worked (306.97). After MANY attempted failures, I was finally able to navigate to the device manager before it froze, removed the problem NVIDIA driver, and then was able to use the computer with the generic
video driver, and very quickly download the specific driver I needed (306.97), and it worked.
Then, I had to disable ALL automatic updates from downloading the more current NVIDIA drivers, because it did this several times, and crashed my computer every time. I just had to work extremely fast to get to the driver and roll back to previous driver
and uninstall the newer one. It's been working now for several days.
On the laptop, which has never had a problem with NVIDIA drivers, I figured it would be easier. But I had another problem. My charging port had an error, which I already knew about, and the install would roll back to Win 8. So, I removed the hard drive,
put it in a laptop I already own and was running Win XP on.
I ran Win 8, and installed 8.1 with no issues. Put the hard drive back into the original laptop, and only needed to call Microsoft to verify the license number (which only took a few minutes). Everything is now running fine. NO regrets. It only took
several days of my life to do this, and the contemplation to throw both computers off my roof.