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Hmmm. Slayer, it's just so bad. You really need to look in some other stuff. Real instruments are always the best, but if you don't play guitar, you should look into some samplers like LPC Prominy, which is just guitar, and Ministry of rock, which has full package of guitar bass and drums. Also for the drums look into superior drummer 2.0.
Now, about the song. The drums need alot of improvment. They start off ok, but then after that they get bad. The guitars play the same thing throughout the whole song, it's very repetitive and gets boring fast.
Don't get me wrong, it's not bad at all for your first try. Just keep trying ;)