You lack details on your query. I shouldn't have to dig for such. NO OS details, laptop or desktop, ram and if old or new system, etc., savvy?
In general terms, if the empty HD space becomes filled with a new HD. Then get the proper cables, etc. and mount it, plus any jumpers. Then boot with the system and access the new HD under the HD management services(google) and format, etc., to prepare the HD for use. Once done, its ready and used as extra capacity. It WON'T be an expansion of the "C" drive but rather a separate drive itself, maybe "E", whatever your setup has open next. If you like, check your vendor or system maker's support website for details on upgrades, etc. to pertain to new/added HD installation. Also, the new HD may come with s/w that allows it installation for ease or preparation, check into that provided it comes in the box. If not, then in most cases the HD maker will provide via its support website the required s/w or details to do so for that model#. Be 100% sure when formatting, etc., that you address the new HD and not the old one.
I have an empty hard drive bay and I want to expand my C drive without erasing Windows 7 or Office or Norton or anything else. How can I do this?