1) Unplug computer from power.
2) Observe anti-static precautions.
Your body carries Static electricity. Static will fry out (Short Circuit) the hardware components inside of a computer.
Touch the metal frame of the open computer case, to relieve your body of static. (Should you get up, and leave in the middle of working on your computer, upon your return Touch the metal frame again )
Alternatively you can purchase an ESD wrist strap, (Electro Static Discharge), and use it. Average cost is around $6. The alligator clip is attached to the metal frame of the open computer case.
3) Identify each cable (Wire) that comes from the Front Panel, and goes to the motherboard.
Make notes, and a sketch of what pins on the motherboard those wires go to.
You may be just replacing the Power On switch, but if any of the other Front Panel cables are accidentally disconnected, from the Front Panel header on the motherboard, you won't have information as to where they go unless you make notes, and a sketch.
[The Front Panel is the plastic panel on the front of your computer, that has the Power On button, Power On light, HarDDrive activity light, etc. The Front Panel header, is the connector, or the pins on the motherboard, where the Front Panel wires go ]
This is HP Support, and the main page for the Pavilion a1600n Desktop PC,http://h10025.www1.hp.com/ewfrf/wc/product?product=3245022&lc=en&cc=us&dlc=en&submit.y=7&submit.x=10&lang=en&cc=us
To see the Front Panel header, there is a photo of the motherboard, and an illustration. On the above main page click on - Product information
Then click on- Product specifications, and finally on - Motherboard specifications A8M2N- LA (NodusM3)
Scroll down the page to the motherboard illustration.
In the very bottom right corner of the motherboard, you will see the Front Panel header, F_Panel 1
The Front Panel will have to be removed to access the Power On switch. This is the time that the cables from the Front Panel, can be easily accidentally disconnected from the Front Panel header on the motherboard.
How to open the computer case, and how to remove the Front Panel, can be found in the Upgrading and Servicing Guide.
On the main page click on Manuals.
Now click on - Upgrading and Servicing Guide - 2.86M. This is a PDF file download. Takes a few seconds to see the first page.
The Power On switch is located inside of the plastic Power On button. The plastic tabs of the button are gently eased to the outside, and the Power On switch is slipped out.
The plastic may have become brittle over time due to age, and heat from inside the computer.
I find using a hair dryer set to low heat, and moving back, and forth across the plastic, helps soften the plastic of the Power On button, enough to remove the PS_ON switch more easily.
This Power On switch should be a good replacement,http://www.directron.com/atxswitch.html