Sounds like a power supply. A 900Mhz computer will not require much. If it is an Athlon/Duron/P3 I'd get a genaric 350-400Mhz power supply, but if is Celeron, then a 250-300 will do fine. You should be able to find something in the $20-25 range that should work.
It is possible that it is the motherboard. You can unplug the 20 pin ATX connector from the motherboard and jumper the green wire to any of the black with a short peice of wire. This should turn on the power supply manually. If it does turn on fine, then it is likely the motherboard.
So you need to jumper between #14 (PS ON) and any of the COM (ground wires like #13 right next to #14). If the PS does not turn on then you know it is dead. The fans and drives should power up. Without power to the motherboard, though it will not boot, so the screen will not turn on either.
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