Let’s see. This is a common problem now-a-days. It may be occur for 4 possible reason.
Your Motherboard better is dead. You have to change it immediately. It's not a big deal to buy a new battery. It only worth 1-2$.
You have a bad power supply. It’s very risky to run your PC with 200W power supply. I can cause a great harm to your PC. You must change your power supply immediately if it's dead. Buy a better power supply like Thermaltake.
You Motherboard is dead. If this happen you must have to change it. If you warranty in your motherboard still now you can give for repairing it to the shop from where you had bought it.
At last the problem is with your ram. It may cause this problem. You can change your RAM and can check it.
Before opening up your PC please discharge your Static by Touching to a metal.
You can try troubleshooting your PSU by opening it up and Locating its Jumper follow this tutorial
If it works then Great, No need to Replace it. Now we can now move to other causes of this situation.
Testing out the Ram (Trial and Error) , Try removing a ram and check if the PC boots up repeat the process until you tested out all the Rams. If the Pc boots up upon removing a ram then you have there a Defective Ram , Buy a New one.
If nothing above still works , lets move on
Check if the wires are properly attached to each peripheral
Replace your Cmos Battery
Check if the thermal paste of the processor haven't dried out if it dried out replace it with a new one by buying a thermal paste or silicon paste and follow this tutorial http://www.youtube.com/watch?v=o7rPqCvCt0g
Because one way of protecting your computer from Overheating the Bios Automatically Turns off the system.
Well if nothing works out maybe you have a Defective MOBO. Now its time to drop by to the nearest computer repair shop.