So far you're on the right track with trying to solve the problem.
Make sure the board has absolutely nothing connected to it. Just a bare board with CPU, fan/heatsink, memory, and video (unless onboard). Next pull the CMOS battery, unplug the power, wait about 5 mins and put it back in.
Very important thing to do that many many people miss is making sure the hardware is making good connections.
Remove the memory from the slot, and clean the base where it connects to the board. Use compressed air to clean the memory slot as well. Do the same thing for the video card if it is a PCI or AGP.
Also make sure only power connected inside is for the motherboard and nothing else. No drives or anything.
If that does not work, then you will need to look into getting other parts to swap to test such as the CPU, memory and video.
If it does work, then assemble the computer back to how it was but only 1-2 parts at a time to eliminate the bad part.
Good luck.
