aplologies for the late reply.
o1die, the power supply brand is Antec TruePower New 650W Modular PSU. I have tried with other power supplies, but this is the one I just recently bought.
davcon, sorry, I don't know what that means! The strange thing is that it was working fine for almost a year, then decided one day "oh, I don't fancy working anymore, I'm going on strike" and presented this problem.
PS, many thanks for your responses and in advance for future ones.