All right first off it would be great to know what the components are.
If you haven't been told if the thing works fine, I'd advise to run some tests and take some readings from the power supply channels with a multimeter just to make sure the juice has still nominal numbers.
Power supplies get old fast, if you don't want to take any readings at least make sure there aren't burnt components. Check for carbon smell and darkened areas with some flashlight.
Then, you want to check if the interface for the HDD<>Motherboard is compatible; it probably is something along the lines of SATA2. You also have to make sure you can power the HDD properly. It's difficult to advise ATM due to lack of details.
Once/if you got the HDD connected and running, you gotta enter the BIOS by pressing an F key at startup (it will tell you which one briefly at the start). You can either install a Linux distro from an USB pendrive or use a Windows DVD, just make sure you enter which unit will the system read first by using the motherboard's BIOS (set priority/order for optical drive, USB port, HDD, etc).
If you manage to get to the OS installer, you're halfway there.-
<message edited by Ingram on Saturday, August 10, 2013 9:39 PM>