The HP Pavilion 503n Desktop PC uses an IDE (PATA) harddrive,
HP Support > Pavilion 503n Desktop PC > Main Support page,http://h10025.www1.hp.com/ewfrf/wc/product?product=90381&lc=en&cc=us&dlc=en&lang=en&cc=us
In the list click on - Product information
Now click on - Product specifications
Lastly click on -
Motherboard Specifications, TriGem 2002052 (Glendale)
Takes you to this page,http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph07813&tmp_task=prodinfoCategory&cc=us&dlc=en&lang=en&lc=en&product=90381
Scroll down, and look at the motherboard illustration, and photo.
Viewing the illustration look at the bottom right.
Primary IDE is where the harddrive plugs in.
In the motherboard photo this is the long Blue connector.
Now lets view an IDE 'ribbon' cable,http://en.wikipedia.org/wiki/Parallel_ATA
Click on the photo to the upper right that shows the IDE cable, and it's blue connector. (The photo on the left of it shows the motherboard connector)
Here you see the rectangular connector, and a small square 'bump' on top.
The bump is a Locating Lug.
This lug is lined up with a cutout, in one side of the motherboard connector.
The other end of the cable's Locating Lug, lines up with a cutout on the back of the harddrive.
IF, there is no Locating Lug on the IDE cable connector;
There is a faint red stripe on one side of the cable. This is Always on the same side as number 1 wire, in the cable.
On the above link scroll the page down towards the middle, and the IDE cable illustration. (Under the heading - Parallel ATA Interface )
Number one contact pin on the harddrive, is ALWAYS towards the power connection, on the harddrive.
The above is for a Western Digital IDE harddrive, and shows the jumper settings.
I posted it for the back view of the harddrive.
Look at the where it states - 40-pin IDE connector.
Now look at the right side where it states - Power connector
When the IDE flat ribbon cable is plugged into the harddrive, the faint red strip faces - Power connector
On the motherboard look at the blue connector stated above.
Looking at it with the optical drive connector above it, (Secondary IDE),
the red stripe goes to the LEFT.
With the IDE cable plugged into the Primary IDE, and the Secondary IDE connector above the Primary IDE connector, the faint red stripe goes to the LEFT.
Need an IDE cable?http://www.directron.com/ultra66.html
For additional questions please post in a Comment.
The plastic front piece of your computer is the Front Panel.
The area of contact pins on the motherboard, that the wires from the Front Panel go to, is the Front Panel header.
If you need a pinout of what contact pins these wires go to, just state in a Comment )