Power problem

Hi, I have a Dell Inspiron 1440 Laptop that doesn't work. When I connect my AC Adapter to it, it turns on by itself and switches off after 3 seconds. Then turns on by itself again, and turns off. It keeps on doing the same thing. It also doesn't display anything on the screen while turning on. I'm thinking that's because it takes about 3-4 seconds before the Dell logo appears during the POST process. It keeps on doing that by itself, I tried plugging it in for about half a day and when I came back, it still does the same thing.

I've also tried having it taken to a local computer repair shop twice. They said that they won't even try fixing it since it has been disassembled by somebody else before. (Maybe my mom took it to a different technician without me knowing) So I disassembled it myself trying to find the root cause. If it's broken then I have no worries totally breaking it. I was able to get the motherboard out, with the fan and processor still attached to it.

When I plug the AC Adapter in the DC Input Jack, the fan starts to spin, indicating that it's turning on, after a few seconds, the fan stops, indicating that it's off. Then it would turn itself back on again, and off, and so on.

I'm wondering what could be the cause of it and if it can still be fixed, how? Thank you very, very much for the replies.