Changing hardware without a fresh install is never a good idea and may result in an unstable system! When you are replacing the motherboard and/or other components which do not match with the previous configuration, you should always reinstall Windows from scratch to avoid any abnormalities with drivers.
You may be lucky that the system seems to be stable, but from my own experience you will always run into (unexpected) issues. I know many tweaker will disagree with this and may have different experiences, but Windows is handling old (incompatible) drivers very badly with all sorts of issues.
