If your new mobo runs on a different chipset (EX: Via KT400 to an NF2 Ultra 400) you HAVE to reinstall OS. The OS is built around the chipset "kernel" information and switching brands/types/features will make OS fail. If the Chipset is the same and the mobo has basically the same features it should be OK. But i advise a reformat anyways...a fresh start is always nice