Computer problem!! Not starting... need urgent solution

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my Desktop pc is not starting.. Normally when power supply is turned on, the mouse light glows up and keyboard LEDs blink at once. But it is not happenning! I have replaced the SMPS for a new one. yet there is no solution. Do anyone know what is the problem? Even before this, when it worked it was delivering very less speed(around 500kb/s usually will be 3mb/s or more) in USB data transfers! anything related to this..? Please help me.. I am afraid i cannot afford a new PC at the moment and i also can live without it!