Before you wipe everything (and yes, Sturm is right about the process) consider whether it might contain something you need. I don't know about your system. But many complete systems arrive with an OS and a few utilities installed and ready to go on the original HDD. They also have a semi-hidden Partition that contains a backup of this initial installation, to be used to restore from in emergencies. The CD that comes with such systems does not have a full backup of the OS. It has a set of software routines that can do the restore for you FROM the hidden backup on the HDD, and NOT from itself!
So, check whether that is how your system was. Do you need anything from that old HDD, or can you get (maybe you have already) a complete OS backup CD that you can restore from if you need to?