So I bought a ASUS P7P55D LE and i5-750. After installing it in the case my PC wouldn't boot. It would start for half a second, then turn off and after 4 seconds turn on again for half a second and turn off again and so on...
I've disconected everything to the point where there was only motherboard, cpu and psu, but it still didn't wan't to boot.
I assumed that either CPU or motherboard is broken so I took it to the service to check it and for them it worked without problems.
So it must be something with my PSU. It's a Tagan TG-900-U95 900W. It works fine with my current build (Q6600@3ghz, GTX560Ti) and before that it worked for few years with E4300@2.7ghz and HD4870.
I can't understand why it won't even start with ASUS P7P55D LE and i5-750.
Does the power supply work as of right now with your other build? If it does then I presume that your new build exceeds the amount of wattage the power supply can provide. Also, forgive me if I'm wrong but I don't think Tagan is exactly a well known brand, I assume it is a cheap local brand that claims it can provide 900 watts but your computer can only utilize around 400-500 watts. How much did you pay for the power supply?
Also make sure you have hooked everything up correctly, including the front panel connectors which can also cause what you have described.