System problem

[Grim666]

I've just built a new system and it has been running ok for the last week. Then last night it wouldn't start. It seems to power up for a couple of seconds, case lights/fans come on, then it switches off. A few seconds later it come on again, then off etc.
I've disconnected the hard drives & DVDs so that its only running the CPU/mobo/graphics card but it won't boot up at all.
Does this sound like a faulty power supply?
Its the 620W corsair and they have a reputation for being reliable.

Any ideas?

 

[Grim666]

I've tried a different PSU and the same thing happens. I tried resetting the CMOS and still no different. It doesn't reach BIOS screen. Does anyone have any ideas? Or if I have to canabalise another computer whats most likely - CPU, graphics card or mobo?