Boot up?

When I switch on my computer I get a message no display signal then my monitor switches to standby mode.

I cannot reinstal the OS because it will not boot up that far

To date I have tried another monitor, graphics card, motherboard and processor all of which work perfectly well on another computer as do the parts I have changed off the troublesome computer but when assembled into the first computer I always get the same message

Any ideas?

Richard Matthews
April 2008