I've yet to see a method that "saves the OS" everytime, but the repair install usually gets the OS back and running.
It's VERY IMPORTANT to unplug USB devices and to not plug in anything you can do without. You didn't tell if you did that.
Also, if you change from PATA IDE to SATA IDE, or change from Normal PATA IDE to anything else, the trouble contines.
1. Unplug USB and what you can.
2. Repair install.
Working with a friend to try and figure out a problem he's having with his move to an Athlon64 setup, and we're both stuck.
The hardware side of the transition went pretty smoothly thanks to some pretty good documentation for the motherboard, etc. The software side of things is another story.
The first time we tried booting the system, it came up fine, and we were told that because of the massive changes, Windows (XP Pro) needed to be activated again. Wanting to do a few other things first, we said no to the activation prompt and rebooted the computer.
Since that time, any time I try and boot Windows, it just stalls at the wonderful black screen before the initial XP loading screen. If we press F8 early enough, we can get the boot menu, but even Safe Mode just stalls in the same spot.
Figuring this has to do with forgetting to remove the nforce2 chipset drivers (neither of us do this very often, so there's probably plenty of things we should have done before starting), I figured we could pop in the XP CD and do a repair operation. Problem is, the XP CD won't boot either. It gets as far as "Setup is inspecting your computer hardware..." and then just sits there until we reboot the system.
Things obviously work, because I'm posting this from an old Knoppix CD I had around. I'm working on copying over the little bit of data that would be hard to replace, to an external drive.
In the mean time, I'm looking for suggestions on things I might try. Right now we'd like to try and salvage the Windows installation, but if it comes to it, plan B is some 64-bit flavor of Linux.