Ouch. this is tough to troubleshoot, not much substantial information..
Hm, a long time ago, I had a desktop PC built with a Hyper Threaded Pentium 4 extreme CPU. If you know anything about P4's.. they run suuuper hot and consumes lots of power, which is probably why Intel went on a crusade of making low-footprint, high efficiency multi core/multi threaded CPUs.
One day as I was removing the heatsink to properly clean it out and troubleshoot one of its many meltdowns, I reseated the heatsink and tried to boot it and had a similar problem you're stating.
It booted.. went on for about 5-10 seconds, then sounded like the power got cut off and powered it self down.. like a sad dying noise.
What I found out after I pulled off the heatsink was this:
1) I accidentally applied a little too MUCH thermal compound. This slightly overspread on the CPU heatspreader.
2) The generous amount of thermal compound produced an air pocket when reseating the heatsink. There was essentially contact in a ring around the center of the heatspreader, but air between the heatsink and the heatspreader directly in the center because of the air pocket. This provided no cooling, thus causing the immediate shutdowns.
If I were you, i'd immediately consult your mobo's manual, and try to identify any errors specified in its troubleshooting section. If there are any unkown beep patterns, lights on the mobo that you can't identify... they could give you information on what is wrong during startup.
You only need roughly a pea sized drop in the center of your CPU of thermal compound.. you can either gently spread it with a clean business card or do the masterful roll and press technique.
But before you tough that darn heatsink.. try to troubleshoot other possible solutions first.