If you believe there is no god then you are an atheist. If you don't believe either way you are an agnostic.
But I am a programmer, not a theologian so I accept my use of terms may well be incorrect. I'm quite happy to accept A Brain in a Vat's point.
So, if not an atheist, what is the term for someone who positively holds the belief that there is no God? I must have dozed off during RE at school.
If you believe god exists, you're a theist.
If you don't, you're an atheist. http://en.wikipedia.org/wiki/Atheist
If you believe there is no god, you're an antitheist. http://en.wikipedia.org/wiki/Antitheist
An antitheist is of course also an atheist.