Forgive me if any of my lingo is incorrect as i dont know a whole lot about computers. So here is the deal, i could turn on my computer and all the fans would turn on and i would get everything of a new start up except anything to the monitor so i got a new cable and that wasnt the problem. then i got a dif monitor and that wasnt the problem so i got a new video card and yet again no change. the last thing i thought it could be was the motherboard so i went to frys and got a new one. And here is where is gets strange. I hooked everything up and pluged it in and the lights on the power supply and motherboard came on but i would not turn on and no fans wouldnt come on. I thought it could be that i got a dud so i brought it back and got a new replacement but still same reaction. When i hook up the old MB all the fans come on and everything but not the new one. Im at a loss of what it would be. Any ideas?

Could be the processor. Was the replacement motherboard identical to the old? If not exactly the same, you will have to do a system repair using the operating system disc.
Might also be the psu that is not working correctly.

are you sure you correctly installed all the hardware? I did the same (resulted in my motherboard to fry)

did you test the power supply to ensure it is putting out the proper voltage?

i've had a similar problem, and after replacing a few parts it has occured to me that it may be the power supply...and it was! so you may check if everything is ok with it...