When I build a new computer, I buy a new SSD and put windows on it. Then I connect the old hard drive and use it for extra space and to back up all my important files.
You can re-use an old HDD as the only drive on a new build, but you have to re-install windows from scratch which involves losing all the old data.
You boot from the windows DVD, then install windows. If will ask to format your HDD, which will delete your old data.
NB the older a HDD gets, the more likely it is to fail