PC restarts while booting

My first step in a situation like this is to reset the BIOS back to factory. Get into the BIOS screen and just set defaults, save and reboot, see if that helps. If not... Look for a setting under hard drive compatibility that has to do with IDE mode, AHCI or combination mode. Depending on what option is currently selected, try the other options. This most likely isnt the case being that Live CD's arent working either, but cant hurt to try it. Sometimes if you load the PC with an IDE type mode enabled, and the system reverts to AHCI, the system wont boot. If this still proves fruitless...

Move on to disconnect all unnecessary hardware from the PC. External drives, extra monitors, etc and try booting again. If it still won't boot, remove any and all peripherals and expansion cards, such as Modems, NICs, video cards (if your PC has on board video you can use in its place), and all but one stick of RAM. Try booting again. If it boots, great, return things one at a time until the problem recurs to find the problem. If it still doesn't boot swap RAM in until you've tried all available pieces. If all pieces are tried, or if you only had one piece, and it still isnt booting, EVEN into a LIVE CD, its time to start looking at possible bad components... PSU, CPU, Motherboard etc.