1) Computer unplugged from power, observe Anti-Static Precautions.
Your body carries Static electricity. Static WILL fry out (Short Circuit) the hardware components inside a computer.
Computer unplugged from power, computer on a table, computer case open.
TOUCH the metal fame of the open computer case to relieve your body of Static.
Also before you take new hardware components out of their anti-static bags, or boxes.
If you leave your computer in the middle of working on it, be SURE to Touch the metal frame again upon your return.
HP Support > Pavilion Media Center a1320n > Opening the computer casehttp://h10025.www1.hp.com/ewfrf/wc/document?docname=c00593575&cc=us&lc=en&dlc=en&product=1818050&tmp_track_link=ot_search
Replacing the Harddrive, (Hard Disk drive),http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00639788&cc=us&lc=en&dlc=en&product=1818050&tmp_track_link=ot_search
The Harddrive is a SATA harddrive. Uses a SATA 7-pin data cable, and a 15-pin SATA power cable.http://en.wikipedia.org/wiki/SATA
Top right photo shows the SATA data cable connector, and cable.
Top right photo below it shows a typical SATA data header (Connector) on a motherboard.
Typical SATA power cable, and it's respective connector,http://www.playtool.com/pages/psuconnectors/connectors.html#sata
IF, your SATA harddrive has a connection for a standard 4-pin Peripheral power cable, (Misnomered as Molex), AND a connection for a SATA power cable, ONLY use the SATA power cable.
It will Burn Up the Harddrive if you use both.
If you are putting a new Harddrive in, you will need to have backed up, (Copied) the info you wish to save off of your old Harddrive, first. This is if the harddrive is not bad, of course.
You will also need the Restoration Disk/s to install a new Harddrive, if you are going to just replace the harddrive with a new one. (Larger, or replacing a bad harddrive)
If you do not have the Restoration Disk/s you can obtain them from HP Support.
In the US call 1-800-474-6836
24hrs a day, 7 days a week.
Disk/s are free. Shipping and handling is not.
$17 for a 5 to 7 day ship, $24 for a 2 to 3 day ship.
Have the Serial Number from the back of your computer tower handy to give to the representative.
(White Service Tag. S/N is Serial Number)
If you are not in the US, here is the Worldwide Support contact page,http://welcome.hp.com/country/us/en/wwcontact_us.html
If you are going to install a new operating system, per se Windows, (Windows 7?), you may have to go into BIOS Setup, and in Boot Order set the first boot device to CDROM, if the disk does not automatically load.