Gateway desktop won't start up

If the cd light comes on and system fan is spinning you have two options... 1. power supply is failing and not providing the proper voltage needed for startup or two the crappy nvidia 6150 chipset has failed.

Here are a couple of links that may help you to diagnose the problem...it could be your power supply is partially failing, but that's just a stab in the dark.
See here and here and here
Good luck!
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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