Computer not booting

Hi everyone! I've had this problem for quite some time now. I'm not sure when the problem started as it was a gradual process: it happened once a month, then more & more often. All I know is that the changes I've done before this started happening were: installed a new, better PSU, upgraded RAM and installed a new fan for my processor.

My computer would not always boot-up, even though the fans start running and the power is on. It would randomly stop either just after starting or after displaying the mainboard logo. It would sometimes start after I leaned the computer case or moved it a little, or there were periods when it would start normaly.

The problem gets bigger. Since my dad knows more about computers, I told him about this issue and we tried to figure it out. The first thing that came to mind was a faulty PSU connector (sorry if I use the wrong terms, english is not my first language). We got the two connectors out and cleaned them, even using a connecting spray. Nothing changed.

Then he checked if the memory sticks were sitting correcly in their slots (1gb ddr and two of 256mb). After puttin the 1bg memory back, there was a burned smell coming from the area. We both thought that memory was gone because the computer will no longer boot with it, so we tried with another 1gb ddr that works on another computer. The computer will not boot at all. We tried with two 256mb memory sticks and it won't boot either. It will only boot with one 256 mb memory stick.

It worked previously with all memories, now nothing, and I really need those because they made a lot of difference for me. We even changed the PSU (the other one showed strange voltage values in bios: one line appeared to be +5 and -61) with no improvement: the same thing happens.

I have an Intel Pentium 4, 2.4 GHz and using Win XP. Virus scans come clean.

Please, please help me!!