So everything powers up? Can you hear the CPU fan running, hard drives, floppy disk, CD or DVD drives? If you can hear things moving when you turn it on then we can rule out that it's not a power supply issue. This is good.
Look in your motherboard manual, make sure everthing you see in the installation set up is installed properly. This may sound a bit irrelavent, but youd be surprised hwo one simple look at a manual can solve the greatest of problems.
Since this is a video problem, I have an idea. since you're clearly able to get onto the internet via another computer, is it at all possible to take photos of your machine? If you can, take pictures of EVERYTHING, and make sure they're close up. This way we might be able to point out something that might be disconnected. I hope you find this information useful.