When you say 'turn it on' you mean you press the power switch on the front, right? Try unplugging the power to the computer, opening the case and unplugging the larger power cable connector from the MOBO. Bend a paper clip so you can short between the green cable and any black cable. Plug in your AC cable to the PSU again and push in the paper clip. The PSU should run as long as you have the paperclip connected. If it does then it could be your main memory. Unplug your PSU AC cord again, take out the paper clip and plug the power cable back into the MOBO. Find yourself a plain pink eraser. Now take out all your main memory sticks and gently rub the edge connector, both sides, with the eraser. This should leave the connector surfaces clean and shiny. Put the memory sticks back in and plug in the AC cable to the PSU and try booting again. If it still doesn't boot my next step is to use a POST card. They cost about $50 but you may have a friend or a computer shop who will loan you theirs. This plugs into a PCI port and gives you information on the CPU chip's Power On Self-Test. Most come with a manual that will tell you what's wrong if it stops short of displaying 'FF' on the card. If nothing shows on the 2-digit display then likely your CPU chip is bad.
