How to reset Nexus One to Android 2.2.1 or 2.3?
You might have more luck installing a third party ROM as it's not always easy to get the original AOSP (Android Open Source Project) running on your device. Having the Nexus One is good for you since it'll be one of the first phones to actually get ROMs.

If you want 2.3, google for the "Oxygen ROM".

Otherwise, I think there's an original Nexus One ROM right on XDA Developers.

I also found another board topic that has several Nexus One Stock ROMs and you can even customize and "bake" them yourself.