Hardware issue with custom built pc

Back story

I'm having a problem with my pc that i built almost a year ago everything had been running smoothly then when windows loaded sometimes the screen would be unrecognisable i would have to reset or power down and power back on 9/10 this would fix the problem although there was probably an underlying problem that i neglected.

My problem
Today my computer froze it was working perfectly then nothing would work ctrl+alt+delete didn't respond the only thing i could do was hold the power button for 5 seconds to turn it off.

once i powered it back on I got no input from my computer nothing on the screen pressing the power button for 1 second usually turns off a computer but this had no effect all drives fans and lights were working but other than that i got no response from my computer.

Attempts to resolve issue

I got no error noise from my computer nor did i get the single beep u usually get.

1)I tried a different monitor but this showed nothing also no bios screen or error messages

2)I started by changing the graphics card with a brand new one that was identical as i couldn't even see the bios screen this had no effect.
3)I tried the graphics cards in the second pci express slot to no avail

4)removed all ram from the computer just to see if i got an error beep but got nothing.

5)Disconnected everything from the motherboard except the power supply and the graphics card and still nothing

I am starting to think that it is an issue with the motherboard or the cpu but i don't know of a way to test either of these if anyone has any insights as to the problem please let me know.

Specs

Running windows xp professional

Hitachi 1tb Hdd
Asus psn-d motherboard
sapphire HD 3650
Intel q6600 with blue orb 2 heat sink
4Gb 800mhz ocz tech ram

 

Hi sorry for not posting sooner but could you give me a reason why the problems im having could be related to the psu as i don't have a spare it would be a costly exercise and i can't see your reasoning for doing this.

 

I have no access to any other spare parts so i will need to buy a psu or a motherboard to test it.

There has been a few issues with sata port's in the past so it does look like a motherboard problem, also the light on the motherboard comes on to signal it is getting power.

I have written to asus to ask for advice i will wait for there response but will probably go with a new motherboard

 

colts17, i hope when you removed everything aside from the power supply and graphics card you left some memory in..

as for psu, buy a tester. they can be had for like 12 dollars if you have a local shop.

 

Yes I tried it with and without ram didn't make a difference.

I just tested the power supply with a voltmetre and everything was fine.

when i plugged the motherboard power connector back into the motherboard and turned it on everything worked fine again the computer posted and windows ran.

an't really explain what happened just glad it all worked out before i bought a new psu or mobo.

Thanks for all the help glad to be up and running.