switch on the PC all lights come on PSU fan comes on graphics card fan & CPU fan works. But wont boot nothing on screen not a thing. I know the CD drive, floppy,hard drive, modem, gfx card, sound card and monitor all work. The only thing im unable to swap for another spare part so i can check it is the motherboard and the CPU.
Another thing to point out is that allthough everything is getting power there is no sound from the hard drive and nothing on screen.

It's probably a bad connection somewhere but to isolate the problem, try this.

Disconnect everything from the motherboard (internal connections)except for the power, memory and video card. Reseat the video card and double check the monitor connection and power up again. You should get video if the video card is good. If you get video, shut down and connect your floppy and power up again, then the Hard Drive ect untill you run into a problem.

Good luck, I'm writing in a hurry. Hopefully you will get a more detailed response but gotta run to a class.

If the PC is a new custom build, then connect only the power supply, MB, CPU, and case speaker. If you hear beeps, install the memory. Same beeps? Memory is bad. Different beeps? Install video card. Same beeps? Card is bad.
If it's an older computer, make sure the memory is securely installed. If that's ok, try different memory. If that does not work, then it's either the CPU or MB.