Computer Breakdown?

My PC Power Light Started Flashing, it was powered down at the time and now wont boot up , I was informed that this could be a faulty power Supply ? as I am unable to boot I cannot run diagnostics, Is there a way of testing the PSU, Its a Tiny with AMD 64 3000+, 2gb DDR RAM with a creative XFI Fatality Sound Module and a ATI Radion X850 PRO Graphics and I Believe The PSU is 450Watt , 250gb hard drive (ATA) And I also have Illuminated keyboard and mouse both connected to PS2.