|"My computer boots up, but the monitors have no picture"|
How do you determine that your PC is booting up? Do you get the usual POST beep and a normal Windows startup audio play?
When you use the plural term "monitors" are you implying you have a graphics card with dual output and have more than one monitor connected?
Emachines are notorious for failures of their cheap low spec powersupplies. You say you tried with a different PSU. Was it a known good unit of higher rated output than the original?
If you take out the ram and try booting do you get any BIOS beep errors? You may have a failure in any of PSU, CPU, graphics card, Motherboard. Disconnect all other devices including drives and any other PCI cards as well as any USB devices. That eliminates those as possible sources of the problem. Leave only Powersupply, Motherboard, CPU with HSF, Graphics, 1 stick of RAM, PS/2 keyboard.
Will it boot? Do you get any BIOS beeps?
Take out the graphics card and try booting. Any Beeps?
Take out all ram and try booting. Any Beeps?
If you get no beeps in any configuration then you may have a CPU failure. Fans will run regardless of a working CPU.
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