The power supply requirements for the 8600GT is a 300 watt psu, so depending on what all you have in your system, like if you have more than 1 hard drive, etc., I'd have to say that it may be the power supply causing the problem.

Nope , just one HDD.

I don't know if this is relevant or not but this is what i remember what happened the day it went to hell:
My sister was using it , it was working fine and then she was done with it and turned it off.Like half an hour or so i tried to turn it on and that is when the problem first appeared.

BTW ,

I have a Firewire card inside.If the PSU is overworking , will removing the card solve it ? What i mean to say , even though it's not doing anything , is the card consuming energy ?

Is that the psu that came with that machine?

Yeah.

Well unless this is a hand built machine it seems strange why someone would slap in a psu at 270v when as Xeno points out the gpu requires a 300v psu.

How obscure.

Well , i think we just made a breakthrough - it IS custom built.So my computer has been dying for the last two years because the people who assembled the computer were idiots.Thanks guys.I'll get a new PSU on Monday