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[carpetsmoker]

Does the PC actually power up? or does it crash? Do you hear the power supply? beeps? harddisk?

Harddisk are the most common cause of crashes.

Check for leaking capicitators on the mainboard ... Happens quite often with PIII and above

Does the mainboard still get power (usually there's a led)?

Take out all RAM modules but one, check if the pc powers, if it doesn't than swap the module with another ... easy way to check if your ram is ok.