The light is flashing green on the rear of the power supply and constant green when it is ok , when it is flashing the power light around the on switch wont come on at all the pc is a hp pavilion t340. uk , 2.6 ghz pentium 4 1024 ddr ram 80 gig hard drive there have been no hardware changes in the last 12 months or any new software installed for the last month or two , I am pretty certain it will be the power supply that is going faulty .