Computer Issues. Any Suggestions?

I have been working 3 jobs recently but that is beside the point. The point is that in the last 3 months my desktop computer has gone on the blitz. I have done my own computer work in the past so I have some technical know how on the subject but being a little lazy I am turning to you guys for some input. I have done a little research already on the subject to make sure of the issues involved but I need a little more specific answer than the generic stuff I have been getting lately. I have a Dell Vostro 200 desktop computer. The power supply on the back is a green light which means it is fine. The issue that I am having is that when I press the power button the fan starts up and 2 seconds later it shuts down with an orange light on the power button. When opening the case I see that the mother board has an amber/orange light instead of the normal green I had in the past. According to the internet the motherboard is bad. My question for all of you out there is which part is bad? The CPU? The motherboard itself? The SIMM chip? or is it the watch size battery that is going bad? I have noticed in the past few months the time on the computer that was on the desktop kept losing time so that I was my first instinct to get it replaced but somebody on one of these chat sites stated to me that it was the motherboard issue and not the other components. I followed the proceedures of unplugging the power cable and draining the power off the motherboard and then starting it back up to no avail. I don't want to purchase a whole new computer and don't mind purchasing some parts since I can do it myself some but I don't want to purchase anything if it is going to be the entire thing.
Let me know what you think.
Cheers,
Izaak