what the problem is with my pc.....when i newly made it, it was running fine...
This got me thinking about another (possible) issue. You state that you built your system yourself? If this is so, I would wonder if maybe your having an issue I have personally experienced. And not just me, most ANY licensed system builder has done this a time or two when in a hurry etc.. Especially if you've built less than 50 systems.
Make absolutely sure you haven't added any standoffs that don't have corresponding holes. This is easier to do than you might think when aligning a board and setting the standoffs prior to assembly. Usually, if this is the case, the unit won't boot at all (direct short), but I've seen systems work with a shorted standoff and then mysteriously "quit" post burn in. Also be absolutely sure you didn't leave any extra screws between the mainboard and chassis (A good shake of the case will usually tell this one).
Boards microsopically "settle in" with heat cool cycles, so this problem is not as uncommon as it might appear. I suggest, follow Nick N's advice and check your Power Supply, if its good, then you might consider checking for orphaned stand off's in the chassis. Other than that, you might need to take it in for a checkup. Could be a bad mainboard, mem / chipset, or CPU causing these issues as well..
Regardless, this is definitely hardware (IMHO) :)
Wish you the best of luck and a speedy resolution
Mick

Athlon 64 X2 (T) 4200+2.2Ghz,1GB PC2-3200 DDR SDRAM,250 GB WD SATA HDD
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