pc not powering up

[lizardking]

hi can any1 help me, my pc has stopped powering up. i shut it down as normal, then the next day it wouldn't work. when i switch it on the fan starts but there's no power going to my drives. the only way i can get my drives working is if i leave the power leads in & unplug the ribbons from the back of them. there is no signal going to my moniter at all. every thing seems to be working apart from when the're connected to the mother board.my OS is windows XP. please help

 

[itsme]

What is the PC Spec?

 

[lizardking]

520mb ram, 160g hd, n-vidia graphics card & i don't know what processor it has.
i've just tried more memory but it's still the same.
it's about 5 years old now so it could just be old age, served me well though

 

[itsme]

Pretty sure its your PSU [Power supply]