MacBook Pro 5.3 power button not working

MacPro 5,3 from 2008 can still run up to 10.11 El Captain, so can still be used for most purposes on the Internet.

If you are especially sensitive about privacy AND stored stuff on your computer that MUST not be learned by anyone, you can remove the internal drive and break it with a hammer.

If it were operable, Zero-ing the drive will suffice for all but Nuclear secrets.

Yes, that will be tricky to remove that screw without causing damage to the Logic Board or the Battery. The least risky method would be to find some sort of screw driver (flat blade, torx, etc.) which can fit into the remaining opening of the screw head to see if it can bite enough to remove the screw....most likely you will destroy the driver(s) in the process.

Make sure to open the Display lid of the laptop and place the laptop upside down on the edge of a table with the display hanging downwards and the keyboard/palmrest lying flat on the table. This will allow you to press into the screw driver to try to get a good bite of the screw without putting any pressure on the fragile display. If the driver can bite, it may turn the screw. If the screw is frozen, then lightly tapping on the handle of the screw driver (with a small hammer or even the handle of another screw driver or small block of wood) while applying some counter-clockwise torque on the driver can sometimes help to loosen a frozen screw, plus it may help the driver bite into the screw head.

Sometimes side cutters can be used to bit into the outside of the screw head to attempt to turn the screw, but I doubt this will work since there is very little room to work. Otherwise grinding a slot in the screw head or even completely grinding the screw head off using a Dremel cutting blade, but this would cause a lot of metal filings which can destroy the Logic Board and other electronics. Plus using a grinding blade is risky on a battery since you don't want to puncture it or it could cause a fire.

Actually cutting the plastic bracket for the screw may be an option since that is somewhat soft plastic, but it would mean you would no longer be able to securely mount this battery in this laptop thus needing to purchase another battery (you may need to anyway if the battery is the cause of the problem). Again, extreme care needs to be taken since a blade can also puncture the battery if the blade slips. Plus you are working with a metallic conductive tool which could possible damage the electronics if the blade slips or it comes into the circuitry which may have some power in it if the battery has any charge on it.

You may want to take it to an electronics or computer repair shop to have a more experienced technician to remove the screw and/or the battery for you. An experienced & reputable tech should be able to remove it without causing any damage to system (except for the screw and the battery bracket).

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it looks like you have a 3.5-in SATA drive in the upper right corner and a DVD readers in the lower right.

Everyman says the 5,3 model shipped with up to 500 GB SATA drive.

OWC says its a nominal 2.5-in form factor drive.

Try disconnecting the battery & trying to power on the laptop. It is rare that the power button fails. It is more likely that the battery is preventing the laptop from power on.