Your best bet may lie in setting your CD drive as the first boot device in your bios, then loading your original windows install disk, and the first thing the computer will do after booting to the windows install disk is to inspect your configuration. And at that stage, it may reset the configurations to be compatible for your new board. Then it will go on to give you an option to exit, or reinstall windows, or repair the windows you have.
You could try exiting at that point and with luck windows will then start correctly. Failing that, I would try that same procedure one or two more times because the first try may not be enough.
But I recently had exactly your experience and thought that there would be no way to save my original configuration with a new mobo, and went directly to the install windows option. And to my pleasant surprise, I was offered a chance to save my original configuration. On the up side, after reinstalling windows, all my original desktop icons and other software were left in tact, and the downside was that all the windows updates were lost. You could try that also, but to be on the safe side and hedge your gamble, you could image your old HD to another storage volume so you have a copy in case it does not work.
And there are a number of yes no prompts, if you do try reinstalling windows, and it asks you permission to reformat the disk, and you say yes, you will lose everything on the disk in the resulting reformat.
And at the end of the day, you will still have to reactivate windows, if windows refuses, you may have to buy a new license. Especially if you can't come up with the original 20 COA letters.