Computer help forum: Gateway GM5478 will not boot.

I have a Gateway GM5478 that has been a dog since day one. When it works it's great. The problem has been keeping it working. It is not my primary pc. I only use it as a home theater set up to watch on line videos (network programs, nfl.co, nba.com and etc.) on my big screen. I have in the past at various times replaced the hard drive, the video card, the memory, the cd drive and most recently the power supply. I also installed Windows 7 and have kept it current. I have virus protection and anti-spyware installed.

A couple of days ago, I turn the computer on and the power light comes on, the fan starts and 5 seconds later it shuts down. I have tried the Gateway recommended power drain, re-seated all the cards as well as the memory simms. I also replaced the power supply. All to no avail. Still the same symptoms.

Can anyone suggest further trouble shooting, or maybe recognize the symptoms having a specific cause.

I do live very near the beach and have noticed the connections on the rear of my TV's to have started showing heavy oxidation.