Well as it is an old one I guess it has been upgraded to XP, so you should have a Microsoft XP installation disk. Just boot from that and take the "repair" option. If you only have a "recovery disk" you will have to follow the HP instructions............I guess there should be a manual on the system somewhere.
Don't forget to run chkdsk first..............if the hard drive is damaged there is really no point in going any further.
Boot into "set up" first and check that the CD is above the hard drive in the boot sequence.