My computer specs are:
Intel Pentium4 660 Hyper Threading 3.6
Gigabyte LGA775
G5 looking Case with Enermax Noisetaker 485w ATX12v v.2.0
1GB ddr2 Dual channel
XFX Geforce 7600GT
2 Dvd drives
Zalman Heatsink/Fan
2 80 mm Fans

A week ago I added a 120mm fan to the outside of this case(on the side) screwed it to the bottom vent, the fan is pluged to a 4pin cord. I didnt didnt have time to test it until the next day to find out that my computer wont turn on, no lights, no fan movement, monitor is black but asks about monitor connected? when i unplug it from the PC. I dont know why there is a problem because theres a 80mm fan in the fan rack in the case plugged into a 4pin cord too, which ran fine.
People tell me its the PSU(485w should be plenty of power to support a extra fan thats 120mm right? ), motherboard or both. I searched the net and read about shortcircuts, and using anti static bags. Which at the time i was sitting on carpet when installing the fan but would any of this be related to the computer not turning on I need to find out why it wont turn on, whats causing this and whats the solutions. It will be Greatful if someone teaches me hows the right way to install a fan.
Please give full details because i am lost :sad:

I sort of noticed when i plug the power cord to the socket, than turning on the switch on the back of the PSU my lamp lights dim for like 1/4 a sec, even with only PSU MB 2ram sticks and cpu fan pluged in it wont turn on

I have a spare 300wat Compaq PSU, its for my amd 1.2 GHz can it be used with my system? I just want to try out if its my PSU

4pin fan connector for a 120mm not a three, those 4 pin are for the cpu fan on intel motherboard. The first thing I would do is check to make sure the the fan is plug in to thr right header and then reset the cmos, see if your computer turn on if not unplug the power supply connector from the motherboard take a paper clip and jump green wire to ground black wire and see if powersupply kick on if it does your motherboard if proably damaged.