Im guessing what you meant to say is, you start the pc and everything gets power, but nothing shows up on the screen. This could be for several reasons:

1. The motherboard is defective
2. The CPU is defective (let it run for a little bit, turn it off, take off the heatsink and check if the core is warm)
3. The RAM is defective (if you use two sticks then try using only one, then the other, if the first didnt work)
4. The cmos needs flashing (make sure you read the manual, but try taking out the battery, wait about 10 seconds, and reinstall it. On some motherboards you also need to move a jumper, so check the manual)
5. The powersupply is defective (try a different)
6. The video card is defective, or not entirely in the AGP slot (try pushing it farther in if you're not sure)

You say the components work individually, but this should hopefully give you a few things to try...

Good luck

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