I suspect general motherboard failure.

Okay. Here's some things to check.

• Are the minimum things needed to boot present? A barebone working system includes a motherboard, processor, a full bank of memory, a video card, and a drive to boot off of. If these things are not there, it will not work. Are these things all properly connected? 
• Try removing everything else one by one and try to narrow down the problem. 
• Are the jumper settings correct? You must set jumpers correctly for processor type, speed, voltage, bus speed, etc. Consult the manual. 
• If you tried to streamline the BIOS, return all settings to default. 
• Is the motherboard in good shape? Any cracks, bent pins? 
• Is there enough power? If you are adding more or newer components to an old system, your power supply might not be powerful enough. A mainstream power supply is 230 or 250 watts. 
• Troubleshoot other hardware in computer. 
• Check around the net and see if there is a known "bug" on your particular motherboard. Check the manufacturer's web site to see if there are any updates for it. 
• If you recently had a bad rain storm with lightning and have no surge protection on the computer, you could also have a fried motherboard due electric surge.

System Loses Time, Settings, or gives Dead Battery errors.

This one's easy...the battery is dead! Replace it.

Battery only works sometimes.

Sometimes the battery works fine. Then, sometimes you get errors, or your setting are gone and the computer forgets what time it is. In many cases, this is because the battery is getting low and is losing voltage. It could also be a bad connection between the battery and the motherboard.

I can't figure out how to get into setup.

Every computer is different on what key combination will take you into it's setup program. It a factor of what BIOS version and maker you have. But, immediately after booting the machine, look on the screen for the keys to hit. In many cases this info is at the bottom of the screen. It will say "Press "X" to enter Setup" or something like that where "X" is the key(s) to hit - usually 'F1' or 'Delete'.

I changed the CMOS settings and it doesn't show.

When exiting the CMOS setup after changing settings, make sure you choose "Save and Exit" to leave. Choosing "Exit Without Saving" my accident will cause you to have this problem.

The motherboard is cracked.

Well, unfortunately, this means you will have to buy a new board. You can't repair it. To prevent it in the future, make sure the motherboard has plenty of support to prevent flexing. And be careful when pressing cards into the board.

There's a bent pin on the board

If its just bent, carefully bend it back with pliers and the power turned off and unplugged. If its actually broken, there is nothing you can do. You will have to buy a new board, or if it still works, live with it.