The melody is great!
The only problem is that your using a computer software to create the sound of distorted guitars and it's very difficult to do so and give it that real ballsy sound you want. The only suggestion would be to spend a bit of cash and get yourself a really high end guitar producing program. I would say the same about the drums too, i hear great things about EZ Drummer, i don't know I've never tried it but I've heard other songs that people have used it and it sounds pretty good. Of course you can never replace the actual sounds the live instruments will give you but hey not everyone can play guitar or drums. But keep experimenting and keep it metal!