I will throw out a couple of things that come to mind.
when I just [power] down, there is still power to the PC, video card is lit up and what no
(Word in brackets is mine). That means that the capacitors are still being charged even when the unit is powered down. Turning off the power supply (I assume you mean the power switch on the back of the PS) cuts all feed to the system and the capacitors discharge. Then your mainboard power-on lamp goes dark on the board, etc.
> First, reset your BIOS back to factory default. I don't think that is the problem, but just in case there is some strange setting or corruption it is easy to do and takes only a couple of minutes.
> I will assume that you do not have a power supply tester; so you have to swap out the PS with another if you can get your hands on one that you know for sure is good. There is a homespun way of using a hairpin to test a PS, but I don't like it because it is dangerous and if you get it wrong you can injure yourself and the PS and void any warranty. Here is a tester that I use all the time, the Apevia, 3rd down from the top. I am not suggesting that you buy one, this is just FYI.
> One last test you might try is your power switch. Pull the tiny front panel lead for your power switch where it plugs into the mainboard. Take a flat bladed screwdriver and short the two tiny contacts on the motherboard. Your computer should power up normally. After it boots and is running, cross those two leads again and your computer should power-down normally. If it starts again and powers down normally, the culprit is a bad front panel power switch. If it does the same thing as it has done, you have a bad motherboard (IF the power supply tests as good). This is not a shocking thing; I have sent several motherboards back to the retailer when I tested them.
Here is a link to a tutorial on building a new unit; it also has information on testing components as you go. You might get an idea or two from it.
Please post back when you are done and let us know how it is going. Good hunting!