hello guys my pc blacks out after few seconds a start up but the motherboard
shows that its receiving power so i don't know the exact problem

If you are running windows then try starting in safe mode. It could simply be your video driver being in the wrong mode for your monitor. If not what OS are you running and how far into the boot does the system go?

Perhaps you should check for loose connections on the monitor and on the cable connecting into the graphics card. If that doesn't work then download the ubuntu live cd then insert the ubuntu live cd and boot from the ubuntu live cd to see if it's a software or hardware falt. If the screen goes black when booting from the ubuntu live cd it means some hardware will need replacing however, if the ubuntu live cd loads fine then you will need to insert the windows installation cd and do a disk repair.

It could well be an overheating issue as well.

Am running windows XP, When booting it takes like a minute without displaying anything and after it blacks out

Try cleaning the dust away on the heatsink and fan. If your PC is old, you might want to apply a new thermal paste between the heatsink and the CPU.

as rch1231 said try safe mode ,hitting f8 on start up ,choose safe mode