You MUST realize that "REFORMAT" means you wipe ALL of the information off the HDD and start blank. So you have NO Windows on it, and MUST re-install from a CD.
To prevent such a disaster, Windows usually refuses to let you Format the C: drive that contains itself.
As photonboy suggests, wiping that old drive clean and then re-installing in the "new" machine with the new mobo IS a good idea. But you need that Windows Install CD to do it.
By the way, an even better way to wipe a HDD and get any weak sectors fixed up is to do a Zero Fill operation on it using some third-party utility - Windows does not do this job.