OK, before I jump in to help, I gotta ask you to shorten your sig. The rules only allow for five lines. You can add most of those specs into a few lines and your all good.
Ok, now for the helpful part. I'm not sure if you have everything in the case or not, but if you do, take it all out. Put the motherboard on the anti-stactic bag it came in to protect it, and to make sure it doesn't short out on anything below it
With ONLY the processor, RAM (one stick), Video card if the comptuer does not have on-board video, mouse and keyboard plugged in, connect power, and short the power jumpers on the motherboard with a screwdriver, or my favorite a pocket knife. If everything works here, then all the main things are working proporly. If the keyboard still does not work, try a different one.
If this works, slowly add One device at a time to the mix (optical drive, hard drive, second RAM stick, what ever) and keep adding devices until things don't work. If you do run into something that doesn't work, throw it on a "Bad" pile and continue to add devices on at a time untill everything has been tested.
At this point if you have tested everything, you know what is bad and what is not so you can put everything good into the case, screw it down, then fire the comptuer up again to make sure it still works.
Now, if that original setup doesn't work, start with a different RAM stick. If new RAM still does not work, try the same sticks (one at a time) in different RAM slots on the motherboard.
If you are still having problems try a new video card. I doubt that's the issue, but it wouldn't be the first time I've seen something like that do it. If the video card is a no go, take out the CMOS battery, and take it to a watch store or hardware store and get a new one. they are maybe $10. While we would all love to think that CMOS batteries are new, by all means they could have been sitting in the warehouse for a year to two before they were put into your board. (if either of these last two paragraphs helped, then go back up and add parts one at a time as described above and ignore the rest of this)
If all this does not work, odds are you have a bad motherboard. Throw on a USB keyboard if you have the option as one last test in the hopes that you just have a bad PS2 port.