I too thought it was the power adapter after reading the posts.
I can tell you for sure, it is not the battery, and it is not the hard drive. I am not going to take up time reiterating everything you said that makes that obvious, but lets just say you already did a process of elimination. So, it is neither of those two items.
Now, a simple check with a meter on the power adapter is no good. You say it powers on for a quick second and shuts off. This would typically indicate a motherboard issue, however, because you said that the light goes out, and only comes back on when you unplug the adapter and plug it back in, it makes me think it could still be the adapter. The test with a meter is no good because it sounds as if the power adapter may be putting out power until a load is put on it, at which point it shuts off. Your best bet is to try a different power adapter because there is no easy way to check the power adapter under a load unless you have more advanced electrical knowledge and/or are willing to disassemble the laptop and check the voltages at the jack while it is plugged in.
Power adapters can be had for cheap. You get what you pay for so there isn't a chance in the world I would buy one of these cheap adapters that will probably melt and burn your house down within a couple of weeks. A good one will cost you around $30 on Ebay. So, it would be my suggestion that you simply take the laptop to a service center and kindly ask them to plug it in to see if it will power up on one of their universal adapters (or equivalent adapter). Seems how it should only take 30 seconds, I don't believe any reasonable, self-respecting technician/company would charge for that. Otherwise, you can purchase a universal adapter from a big box store and test it out, and take it back if you decide it doesn't work.
NOTE: If you get the same results with a known good adapter you are probably looking at motherboard failure. However, you cannot be sure until you unplug all devices that could be causing an issue, such as memory modules, hard drives, cd-roms, wireless adapters, modems, etc. These all can usually be accessed and removed quite simply in order to see if it effects the ability of the motherboard to power on. It could be a bad device plugged into the motherboard causing all your issues.
Edited by appleoddity, 02 April 2011 - 08:06 PM.