i assume you mean you want to build a new box but carry over your operating system and everything right?
in that case, before you do the final shutdown of your old machine, go into device manager and delete all your "system devices"...(these are the things associated with your current motherboard)
then, build up your new system, and install the hard drive...on first boot up, windows will find ALL your new hardware and install it all, but it'll take awhile.
you may eventually want to reformat, but this method should get you back up pretty quick.