Basically I built my computer about 2 years ago. Never had any problems with it. Left the house yesterday after having used it and it been fine got home today, turned it on and its now not working. The case fans, cpu fan, gfx card fan etc all power up. After about 3 seconds the led's on my keyboard flash. However this is as good as it gets because before anything even registers with my monitor everything powers down again. It stays off for a second or two then starts to boot itself again. This process repeats over and over. I've tried testing each stick of ram individually as well as a different hdd but the exact same problem continues. I've had a look around various forums but the only information relating to this kind of problem talks about rigs that are in the process of being built.
Can anyone shed any light on this? I'm thinking maybe an issue with the PSU but thats a complete guess! Any help will be massively appreciated.